The marriage of Commons Speaker John Bercow was in crisis last night after his wife Sally was accused of having an affair with his own cousin.

The Mail on Sunday understands Mrs Bercow embarked on the relationship with Alan Bercow, a lawyer, at the start of the General Election campaign.

While the Speaker campaigned in his Buckingham constituency, Alan Bercow moved into the £1.2 million mews house in Battersea, South London, which is owned by John and Sally Bercow.

Mr Bercow, 52, remained in his grace and favour apartment in the House of Commons, but his three children with Sally are said to have remained in the Battersea flat during the affair.

It is understood that after a series of arguments with Sally, 45, Alan moved back into the house he shared with his wife Erica a week ago and that the affair is now over.

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Sally Bercow (pictured) - the wife of Commons Speaker John - has been accused of having an affair with her husband's cousin

Sally's affair with Alan Bercow (right) began at the start of the General Election campaign and continued while John (left) was fighting for his Buckingham constituency, Mail on Sunday understands

Alan Bercow, 57, is a commercial litigation partner with the law firm Stephenson Harwood, and is co-head of the company's fraud and asset tracing group. He is the son of Samson Bercow, who is the brother of John Bercow's father Charlie Bercow.

A description on the company's website says he is 'particularly experienced in obtaining and defending interim injunctions, including worldwide freezing injunctions, disclosure orders, anti-suit injunctions and gagging orders'.

A family friend of the Bercows told The Mail on Sunday that Alan Bercow, who has a son with wife Erica, 'became close' to Sally Bercow due to 'a mutual appreciation of fine wine', a subject on which he 'decided to instruct her'.

Speaking at the door of her £3 million house in the exclusive suburb of Chiswick, West London, Erica Bercow confirmed that the affair between her husband Alan and Sally Bercow had taken place.

When the MoS put it to her, Erica, a lawyer, said: 'Yes, but I can't make any comment, we have our child here. Someone has to be dignified. We are very much together and that's all I want to say. Thanks for letting us know [about running the story].'

Australian-born Erica was at the family home with her husband Alan and their ten-year-old son.

Sally has been involved in controversial incidents before, like when she was pictured kissing professional dancer Clinton Oliver (pictured) at a party hosted by Calum Best

Alan Bercow's wife confirmed that he had been involved in an affair with Sally (left)

Sally Bercow (pictured) met John at a Conservative student conference in 1989 and they married in 2002

A friend of John Bercow told this newspaper that he and Sally were still living separately in the wake of the affair. 'She is still in Battersea. He is in Speaker's House. At the moment there are no plans to alter that arrangement'.

Sally Bercow has described herself as 'the Carla Bruni of British politics', a reference to the glamorous consort of former French President Nicolas Sarkozy.

She certainly was never going to fit the mould of a traditional politician's wife, let alone one married to the Speaker of the Commons. She has courted both publicity and controversy, occasionally sparking outrage.

She met John Bercow, then a rising Conservative star, at a Conservative student conference in 1989 and they married in 2002.

It was not long before she switched political sides, however and, having been a Conservative activist, began to make no secret of her Labour sympathies.

A regular Tweeter, she first hit the headlines in 2009 after using her Twitter account to label David Cameron 'a merchant of spin' and a year later stood unsuccessfully as a Labour candidate for the St James's ward of Westminster City Council.

But it was her appearance in a newspaper, in 2011, clad only in a bedsheet, with the House of Commons in the background, that truly established her in the public eye, prompting widespread criticism.

Mr Bercow (centre) remained in his grace and favour apartment in the House of Commons, but his three children with Sally are said to have remained in the Battersea flat during the affair

She was quoted as saying it was 'sexy' living in an official residence near Big Ben, and added that power was an 'aphrodisiac'.

John Bercow was reported to have been furious and to have privately 'read the riot act' to her after the photo was published. But she later claimed: 'It was just meant to be a bit of fun.'

Just a few months later, it was revealed she was to be a contestant in Celebrity Big Brother – apparently again in defiance of her husband's wishes.

Sally conceded he hadn't wanted her to take part in the reality TV show, but said she wanted to raise money for an autism charity – one of their children is autistic – and 'stick two fingers up to the Establishment'. She was the first to be evicted but raised £100,000 for charity, and claimed she never had any regrets.

But in November 2012, she ran into serious trouble when she sent a message to more than 50,000 followers on Twitter asking 'Why is Lord McAlpine trending?' *innocent face*'.

At the time, McAlpine, a former Tory politician, had been wrongly linked to child abuse. Bercow was forced to suspend her Twitter account and was ordered to pay libel damages in the High Court, understood to be £15,000.

If anything, it seems her early life was more colourful still. In 2009, she confessed to having behaved recklessly: binge drinking, and having ill-considered casual sex with men she'd just met.

'I was an argumentative drunk, picking arguments with my bosses over stupid things.

'Plus, I'd lose my judgment and put myself in danger … I'd get into unlicensed minicabs in the early hours. All the things we'd tell our daughters not to do.' Her flirtatious behaviour still arouses concern amongst her husband's allies.

Sally befriended bare-knuckle boxer Paddy Doherty when she was in the Big Brother house in 2011. He was the 'star' of Channel 4's other reality television show My Big Fat Gypsy Wedding.

Sally Bercow always claimed the relationship was entirely innocent.

She later took the Irish traveller and his wife on a tour of Parliament and Mr Bercow subsequently joined them for a double date.

And last year she was in trouble again after she was caught by The Sun newspaper, kissing Clinton Oliver, a black, tattooed fitness fanatic, at a West End night club.

The Speaker's wife was wearing a brown wig at the party, hosted by Calum Best, and witnesses claimed they exchanged heady kisses, with tongues, lasting minutes.

In a defence of her behaviour, Sally said: 'Of course I'm still together with John. I don't need to apologise. I've done nothing wrong. I went out for a friend's birthday party. I love my husband. He is the most important man in my life.'

Sally has come to be seen by many as an embarrassing figure at Mr Bercow's side. The fact she is 5ft 11in and he is a diminutive 5ft 6in has done nothing to promote a sense of dignity.

She was the eldest of four children. Her father died when she was just 16 and she used her inheritance to send herself to the exclusive Marlborough College – the same school attended by Samantha Cameron. She went on to Oxford to read theology, dropping out after two years.

John Bercow has repeatedly stood by Sally, saying: 'The obligation of impartiality does not apply to my wife who is not my chattel.' Last night, however, his spokeswoman declined to comment.

When Mrs Bercow was approached at her Battersea mews house, she declined to come to the door. A woman, who answered the door and identified herself as Sally's mother, said: 'Sally's not available at the moment'.