Nigel Farage has systematically lied about an affair with a vulnerable former aide spanning more than a decade, The Mail on Sunday can reveal.

The former Ukip leader is accused of breathtaking hypocrisy by his former lover, who claims she was told to keep the relationship secret at all costs – to help save Brexit.

In an explosive interview, Annabelle Fuller, once a speechwriter and trusted adviser to the party and its leader, says she and Farage, a married man more than 17 years her senior, had a sexual relationship dating from 2004 until October last year.

Farage, a Member of the European Parliament, has himself spoken of the 'betrayal' of family values in British society. In a series of devastating admissions about their time together, Ms Fuller discloses:

Annabelle Fuller helps Nigel Farage as he speaks at a Ukip meeting in Windsor

Sexual encounters in MEP offices in the European Parliament as recently as last year

A secret love letter in which Farage declares his devotion to her with 'h eart and body '

That the affair helped propel her into depression, self-harm and suicide attempts

That Farage's wife Kirsten – now estranged – was behind Ms Fuller's dramatic ejection from a Ukip celebration party in 2014.

Ms Fuller's decision to speak out is all the more remarkable as both she and Farage have repeatedly dismissed rumours of a liaison. Farage has described the claims as 'malicious and cowardly'.

Within the party, however, knowledge of the relationship appears to have been widespread. One MEP, speaking on condition of anonymity, said: 'It wasn't just a fling or a one-night stand – he was thinking about leaving his wife for her. I have spoken to Nigel about the relationship in the past and know Annabelle and Nigel were still involved sexually until late last year.

'Everyone in the party knew they were having an affair.'

It is also a relationship starkly at odds with Ukip's consistent promotion of family values.

Today, in a perceptive and at times troubling account of their deception, Ms Fuller describes how the affair helped drive her into mental illness and life-threatening self-harm before a successful course of therapy. She believes that only by telling the truth will she be able to put her life 'back on track'.

'Right from the beginning, lying about the affair was a strategy we decided on,' she says.

'We had many conversations about it, even in the past couple of years. He told me I had to keep quiet. I said to him, 'Do you have any idea how painful it is for me?', and he would say 'Yes'.

'Nigel and I both knew we had to keep quiet to save Brexit. We are both liars and hypocrites but the reason I had to lie throughout the years was that I didn't want Ukip or the cause we were fighting for to be damaged.

'He said, 'They will use anything they can to stop this, they will use anything they can to stop me. We are going up against the Establishment here. They will come after you.' '

Although the relationship was consensual, Ms Fuller believes Farage should have taken more responsibility for his actions.

She describes the relationship as grossly inappropriate, particularly in recent years when, she says, she was in a state of deepening mental illness, later diagnosed as post-traumatic stress.

Speaking on condition of anonymity, a close member of her family expressed anger at the way events played out, saying: 'Nigel has acted appallingly with regard to Annabelle. In 2005, he seemed to care for her but by last year he was treating her as an afterthought. Despite her nearly committing suicide, he still wanted her to be his sexual partner as late as last year. He is a complete narcissist and I'm pleased Annabelle can finally leave him in the past.'

Annabelle Fuller began working for Farage after completing a master's degree in international relations

There was a time when Annabelle Fuller was a highly regarded member of the Ukip team; someone who, at the height of her career, would advise Farage, brief the media and write articles for national newspapers under her boss's name.

Today, she works behind the bar of a country pub near Bath.

It has taken a year of therapy to reach the point where she can speak openly about a relationship which began in 2004 in Brussels, where an up-and-coming Nigel Farage was a Member of the European Parliament. Then 23, Annabelle, who had just completed a master's degree in international relations, had started working as a Ukip parliamentary researcher.

'I was introduced to Nigel in the second week of the job and he was polite but cool,' she recalls.

'Looking back, I don't recall an immediate spark of attraction on either side. To me, he was just another middle-aged man in a suit.' After work, like hundreds of others, she would end up in one of the bars around the Parliament building, the kind where Farage held court. And she found herself drawn to the older, charismatic politician. Soon, the two were spending a lot of time together.

'He was fun, with a wicked sense of humour,' she says.

'He had the best jokes and easily the most interesting political stories. I loved listening to him. I was captivated by his worldliness.'

For several months, the friendship remained platonic, with Farage something of a father figure. The vulnerable young woman – with a teenage history of bulimia, concerns about body image and self-esteem – came increasingly to rely upon him.

'He took me into his confidence about work and party matters and would often ask for my advice. I was young but I was bright and I was eager to get stuck in.

'I did research for him – advising him and briefing MEPs on how they were voting. I also helped with his speeches and parliamentary amendments. Our first kiss took place in Brussels in 2005. It was on my forehead. He was just saying goodnight, but it felt very comforting. It didn't feel improper.

'Our relationship grew very slowly over several months. After a while, he took me into his confidence – personally and politically.'

Farage had been with his wife, German-born Kirsten, since 1999, but he told Ms Fuller that it was a marriage in name only.

'He said that it was an unhappy relationship and had been for some time. He told me the marriage was not a real one.'

Then, in April 2005, Farage asked Ms Fuller to work on his – as it would prove – unsuccessful General Election campaign in Thanet South, Kent, as a volunteer.

As their relationship grew, Farage told her it had to remain a secret but they were a 'proper couple' and the Ukip leader moved into her flat in the Maelbeek area whenever he was in Brussels

'I was flattered. Nigel visited every couple of days while we campaigned and sometimes stayed overnight in a hotel in Broadstairs.'

One evening, Farage invited a group to dinner at his hotel after a day of canvassing. As the evening went on, just three of them remained – Farage, Ms Fuller and one other person.

'Nigel offered us a sofabed to sleep on because there were no taxis, but I was the only one to take up the offer,' she recalls. 'I knew what would happen if I stayed and I wanted it to. When we got upstairs I took one look at the room and said, 'but there's no sofabed,' and we both just laughed.'

It was not, perhaps, the most romantic encounter.

'I told him I needed something to change into and he produced a white T-shirt from his suitcase. Then I went to the bathroom, took off my make-up and used his toothbrush. When I came back into the bedroom he was in bed wearing a white T-shirt and white Y-fronts. I got in and said 'budge-up'. It was very English.

'It was gentle. Our clothes came off in the dark. He said, 'you're so beautiful'. We drifted off to sleep in each other's arms.' The following morning, he was up early, as ever.

'He asked how I was, and said, 'Do you regret it?' I said no and he said neither did he and that my a*** had looked amazing in the moonlight.'

The relationship grew, but as it did so, the terms became clear: Farage told her it had to remain a secret.

'He said he wanted me to go into a relationship with my eyes wide open. That he really liked me but he warned me that if people found out they would make my life hell.

'I was already committed. I knew what we had was very special. I was in love. I was happy.'

From then on, she says, they were a 'proper couple' and Farage moved into her flat in the Maelbeek area whenever he was in Brussels.

'He kept clothes and some shoes there. We still went out with others as a group and I think everyone knew by then that we were an item.'

For a while, Ms Fuller happily played the part of a surrogate wife, organising his life, both domestic and professional.

'He was rather messy and left wet towels on the floor for me to pick up every morning. He could also be very absent-minded at times.'

Much later in the relationship, for example, he complained of earache while attending the 2014 party conference in Torquay.

'I found out it was because he had left a broken Q-tip stuck in his ear for days,' she says.

He was never too absent-minded for one important task, however.

In a letter to Fuller, Farage said: 'I only wish that I had been free for you. With all my heart and body. Love Nigel

'He was religious about polishing his shoes,' she says.

'And he seemed to have an awful lot. He told me he usually did the job himself on a Sunday.'

For a while, she was happy to remain a mistress, but as time passed her feelings changed.

'In those early days, he was my reason for getting up in the mornings. Whenever Nigel left to go back to England, it was horrible and I was desolate.

'He couldn't even call me on the phone because he said he was with his family. It was very hard for me to know that he had another life.

'Kirsten knew he was seeing me because someone in the party told her. I used to ring the home in Kent and she would hang up on me, or I could hear her hand the phone to Nigel with an acerbic 'it's her'.'

Growing increasingly depressed by her position as 'the other woman', Ms Fuller says she started to self-harm.

'I started to cut myself some months into the relationship because I couldn't articulate how I felt. When I told Nigel, he was devastated and blamed himself. He begged me to stop and said that he loved me.'

In the summer of 2006, Ms Fuller moved back to Britain to work in Ukip's London press office where, although she still struggled with the clandestine nature of the affair, the two continued as lovers.

On November 3, her 25th birthday, Farage gave her a diamond pendant as a token of their feelings for each other. When that was stolen, he paid for a sapphire and diamond replacement.

But by the autumn, Ms Fuller decided to end the relationship and told him as they went for a walk on Beachy Head in East Sussex.

'I was quite stand-offish with him and said that it wasn't working. I didn't want to be the other woman any more. I loved him but I wanted a proper relationship.'

He said he understood, but that his children were his priority.

A week later, she received a letter from Farage – dated November 25, 2006 – which she still keeps.

'No one will ever love, understand or care for you the way that I do,' it reads. 'I adore you and the bond of friendship that we have shared. What I dread is not being close to you… I fear that this may not go away for many years.'

Pleading with her to take care of herself, he signed off the letter with: 'I only wish that I had been free for you. With all my heart and body. Love Nigel x'.

She says: 'I was angry. He was telling me he loved me and I wanted it to be a clean break.

'I wanted a shot at a normal life. It was the first time he had declared his feelings so openly. But it was too little too late. I had to end it for me and for him but I know it hurt him very badly. We kept in touch because I could not imagine him not being in my life.'

She embarked on a new relationship, but this foundered. And by December 2007, she was back in a sexual relationship with Farage – which continued even when she quit the party in the autumn of 2008 to work with military charities in the UK.

'I'd hoped to make a fresh start but even then I found I couldn't cut the ties to him. He was the only person I could rely on.'

Besides, she found that she missed the excitement of politics and, in 2010, she asked Farage if she could go back and work for him, helping his successful campaign to become leader of the party, before working as his press aide.

At Christmas, he bought her a striking blood-red Vivienne Westwood coat from Harrods.

On her 25th birthday, Farage gave her a diamond pendant as a token of their feelings for each other. When that was stolen, he paid for a sapphire and diamond replacement.

By now, however, there was a further complication. Farage had met a French woman called Laure Ferrari, 16 years his junior. (For years, Farage denied they were together, although in recent weeks his aides have confirmed the relationship, as first reported in The Mail on Sunday.)

Ms Fuller says she and Farage kept their relationship strictly professional between 2010 and 2012, however the sex resumed from 2013 onwards – only this time she felt there was a more sordid aspect to the liaisons.

'He used to ask for hand massages and say he had all the stress in his hands. I used to give him them and we would talk and sometimes he would say, 'how about here', pointing down at his trousers.'

Then, in March 2014, came a bombshell intervention on the floor of the European Parliament when then Ukip MEP Nikki Sinclaire used parliamentary privilege to allege that Ms Fuller was 'Farage's mistress', a claim he refused to answer immediately.

He later branded her action 'cowardly and malicious'.

'I have spoken to every national newspaper on this issue and the answer remains the same as it was in 2006,' he said. 'The answer is no.'

In May 2014, Ukip caused a minor earthquake by winning 24 seats in the European elections, beating both Labour and the Conservatives, a result which left Farage and his colleagues euphoric.

For Ms Fuller, however, it would prove a turning point.

Although she had worked on the election campaign, she was ordered to leave a celebration party near St James's Park in Central London.

'I was told by one of Nigel's security staff that I had to leave on the orders of Kirsten Farage and that she would have me dragged out by my hair if I didn't go,' she says. 'So I did, so as not to cause a scene. It was the culmination of how utterly cut off I was. I felt ashamed, useless and alone.'

She says the humiliation helped trigger a suicide attempt, the first of four dangerous episodes of self-harm in the course of the next few years. 'I went home and slit my wrists and burnt myself with a cigarette repeatedly on my arm.

'I went to hospital after a friend called an ambulance.'

Even so, the sexual encounters with Farage would continue.

'He once turned up at my house in Wiltshire in 2015, a baseball cap pulled down over his eyes,' she says. 'He wanted sex. I told him no. But as usual Nigel was very insistent. We didn't end up having full sex, but I did end up feeling like a hooker after he left.'

And when he propositioned her in an MEP's office in Brussels in the spring of 2016, she once again accepted. 'He came in and shut the door. He was lying on the sofa, we were talking and he said he was horny.

'Very explicitly, he said, 'I'm so stressed' and asked me to give him a hand massage, which was always followed by me performing a sex act on him. It was over and done with quickly. At that point I was beyond caring.'

The last encounter took place in Strasbourg on October 4 last year. Again he entered the office when she was alone. 'He gave me a kiss. I wanted affection. And I did as he asked.

'But I felt disgusted with myself. The next day I learnt the EU was investigating some expenses and I flipped. That combined with the Nigel situation left me feeling that life was pointless. So I cut my wrist in the ladies' toilet of the European Parliament and ended up in hospital in France.'

Throughout this time, she says, the pressure to maintain the lie became unbearable – the more so since they had been 'outed' on the floor of the European Parliament.

'Nigel said we could keep it out of the media if both of us kept denying it and warned that it would be much worse for me if it came out.

'I know we had many conversations about it in 2015 and 2016 when we were standing in my garden in the West Country. He told me we must keep any controversy out of the headlines until after Brexit.

'The other month someone called me damaged, and that's how I feel. A stupid girl.

'I feel used and discarded. Now, I want to tell the truth so that I can claim my life back.

'I'm fed up with living in the shadow of my former self. I'm tired of being ashamed of who I am. I'm not vain enough to think 'He couldn't help himself,' but I do think it was up to him to not to get involved with me.

Last night, Mr Farage declined to confirm or deny that he had had an affair with Ms Fuller. In a statement he said: 'At the time of Ms Fuller's employment the party did not know there was a history of mental illness and other serious personal issues.

'I always tried to help her, recognising that she had ability, and prevented her from being fired on several occasions.'

In his book, The Purple Revolution: The Year That Changed Everything, Farage credits a teacher at Dulwich College for instilling in him a rule for life, 'the 11th commandment: don't get caught'.

Today, that commandment lies shattered. And Farage, Ms Fuller says, has only himself to blame.