A British-based dissident is being pursued through the courts by the UK’s elite crime agency over a plot to assassinate the former Saudi king in a hit ordered by the ex-Libyan leader Colonel Gaddafi, The Independent can reveal.

The National Crime Agency (NCA) has targeted Professor Mohammed al-Massari in a tax inquiry to retrieve £600,000 he is accused of receiving from the Libyans for his alleged role in the audacious plot, which he vehemently denies. The plan was to kill the former King Abdullah by firing a missile at his motorcade.

The Independent can also disclose that Scotland Yard interviewed Professor Massari in 2014 during a criminal investigation into the alleged conspiracy. The Saudi dissident has lived in London since 1994. The action against Professor Massari and another UK-based Saudi dissident comes almost a decade after details of the murder plot first emerged.

The timing has fuelled suspicions that counter-terrorism officers acted against the two men following pressure from the Saudis, who are controversial allies of Britain in tackling global terrorism but have been outraged by the London activities of the prominent critics of the Riyadh regime.

Documents reveal how London – with its large community of politically connected expats – allegedly became the recruiting ground for the plot, which is said to have stemmed from a public row between the two Arab leaders at a summit.

Gaddafi ordered his intelligence chiefs to find a way of killing Abdullah after the row caught by television cameras was played across the Arab world. Libyan intelligence bosses allegedly paid hundreds of thousands of pounds to the British-based dissidents to find people inside the kingdom to carry out the killing.

Abdullah was Crown Prince at the time of the plot, but considered the most powerful figure in the kingdom as his half-brother King Fahd was ailing. He succeed to the throne in 2005 and was in power until his death last year.

Met Police letter to dissident Dear Professor Doctor al-Massari, I write to advise you of our wish to interview you in connection with an alleged conspiracy to murder the now King of Saudi Arabia (and related money laundering offences) which occurred in 2003. We wish to interview you under caution at a police station. (5 September 2014)

The murder plan only came to light after an American middleman, Abdurahman Alamoudi, was stopped at Heathrow airport in 2003 with $336,000 (£238,000) in his luggage.

Alamoudi later confessed to his part in the plot as part of a plea deal and is currently serving 23 years in a US jail for illegal dealings with Libya.

The diplomatic ramifications of the case were considered so grave that the names of the two British-based dissidents were suppressed in US court papers for a decade. The secret court documents detailing their alleged involvement were only unsealed last year.

As part of a plea deal, Alamoudi gave two statements to Scotland Yard which placed Professor Massari and another London-based dissident, the surgeon Saad al-Faqih, at the heart of the conspiracy. In one of the statements, Alamoudi claimed Professor Massari secretly flew to Libya to meet Gaddafi. He also details a series of secret meetings at London restaurants and hotels where money was handed over and details of the plot discussed.

Both London-based dissidents have denied taking part in the assassination bid. They also deny meeting and taking money from Libyan officials.

They claim that Alamoudi made up the allegations in return for having his sentence shortened. The other main co-operating witness, a Libyan intelligence agent, has reportedly been pardoned by the Saudi authorities.

The plot was being uncovered as Libya accepted responsibility for the bombing of the Pan Am flight over Lockerbie and agreed to pay compensation. A few months later the former Prime Minister Tony Blair travelled to Tripoli where he hailed a “new relationship” with the former Libyan leader and claimed Gaddafi was willing to join the UK in the fight against terrorism.

UK court papers show that the NCA is currently seeking to recover more than £600,000 allegedly paid to Professor Massari by the Libyans to find Saudis who might be willing to kill King Abdullah. The police move to seize his assets comes despite the fact the Crown Prosecution Service is declining to bring charges against him and Dr Faqih after Scotland Yard’s earlier criminal investigation.

The row that provoked the plot Muammar Gaddafi ordered the hit on Crown Prince Abdullah after a public row at an Arab summit in Egypt in March 2003, according to court documents. Gaddafi, detailing concerns about US troops in Saudi Arabia, said: “King Fahd [the ailing ruler] told me that his country was threatened and he would co-operate with the devil to protect it.” Abdullah responded saying that Saudi Arabia was a Muslim country and “not an agent of colonialism like you and others”. Wagging his finger at Gaddafi, he went on: “You, who brought you to power? Don’t talk about matters that you fail to prove. Your lies precede you, while the grave is ahead of you.” Abdurahman Alamoudi, the US middleman in the plot, claimed that he met Gaddafi in Tripoli three months later when the dictator spoke at length about his hatred of the Crown Prince and referred to him as a pig, according to his statement for Scotland Yard. “Gaddafi then instructed me that I must tell [British-based dissident] Saad al-Faqih to kill Crown Prince Abdullah. This was to come about in one of two ways: either a personal attack, or a revolution which would overthrow the Saudi regime. This was the first time that I had been told of the plan to assassinate the Crown Prince.”

Prof Massari, 69, a former King’s College physics lecturer, described the claims he was involved in the assassination plot as “absurd”. “There’s nothing in the bank. They want the house. I thought that they wanted to close the file. Now I’m thinking it may be political,” he said.

Dr Faqih said: “I’m aware that they [British police] have been informed by the Saudis of allegations, and they have neglected or ignored that information for 10 years.”

He added: “Apparently the British police and also the British Establishment are convinced that none of these allegations are true.”

Police allege that both men were approached in London in 2003 by the Libyan intelligence agent Colonel Mohammed Ismael and Alamoudi, who was recruited to help Gaddafi exact revenge following their summit row in 2003. The Saudi Prince told the Libyan leader “your lies precede you and your grave is in front of you.”

Alamoudi later told investigators: “The plan was to get the dissidents involved initially in disruption. After they accepted money, they would then be pressurised into providing details of persons capable of carrying out violent actions within Saudi Arabia.”

In a second statement he said “action” and “disruption” referred to sabotage and they were seeking people “willing to be involved and carry out significant acts of violence, sabotage and, as the plot developed, murder and terrorism”.

He claimed he gave the London-based dissidents up to $1m in return. He also claims he introduced the senior Libyan intelligence agent Moussa Koussa to Professor Massari. Alamoudi claims Mr Kousa offered the Saudi dissident weapons and stressed the main target was Crown Prince Abdullah. In his statement to Scotland Yard, Alamoudi said: “Massari was happy at Gaddafi’s request to assassinate Crown Prince Abdullah. He said it could be done, but it would be difficult”

As a result of the London meetings, Col Ismael travelled to Saudi Arabia in November and delivered $2m in cash in bags to a hotel room in Mecca where they were to be collected by the would-be assassins.

However the Saudi authorities were alerted to the plan and made arrests. Col Ismael, who fled to Egypt, was arrested and sent back to Saudi Arabia where officials said he confessed his role and has since been interviewed by US investigators.

Although the NCA’s tax action against Professor Massari continues, Scotland Yard’s criminal investigations into the two British-based dissidents have been dropped. The British government even backed Dr Faqih’s successful application to be removed from a UN sanctions list for allegedly supporting al-Qaeda.

Observers believe his removal from the list infuriated the Saudi authorities who then renewed their pressure on the UK to act against both men.

Saudi Arabia has been suspected of targeting the men in the past. Dr Faqih was stabbed in the leg by two men who forced their way into his London home claiming to be plumbers in what he claimed was a kidnap attempt in 2003 by the Saudi authorities.

Professor Massari fought off attempts to deport him from Britain in the 1990s after the Conservative government admitted that top Saudi officials had brought pressure to have him expelled.

Professor Massari denies he received money from Libya, and the sums of money that the NCA allege he received were from family funds. He said that he was told by police they wanted to interview him just to “tidy up the files”.

The Saudi Arabian embassy in London did not respond to a request for comment.

Plot and counterplot: The main players

Moussa Koussa: A Libyan foreign intelligence chief, Foreign Minister and one of Gaddafi’s closest confidants. Expelled from Britain in 1980 after publicly threatening to kill Libyan opposition figures living in the UK. Credited with identifying Islamic extremist groups which later became al-Qaeda, which led to Libya issuing the first international arrest warrant for Osama bin Laden. He fled Libya and flew to the UK in 2011. He lives in Qatar.

Mohammed Ismael: Colonel in Libyan intelligence services. Trusted by Abdullah Senussi, overall head of Libya’s intelligence services before Gaddafi’s overthrow. He is said to have facilitated payments to Saudi dissidents in London to take part in the assassination plot. Arrested in Egypt in November 2003 after the plot unravelled. He returned to Saudi Arabia and confessed his involvement but received a royal pardon. Now believed to live in Saudi Arabia.