The BBC has discovered evidence that a Conservative minister in the 1960s provided information to communist spies in return for money.
The files - found in the archives of the Czech Security Service - show that Raymond Mawby MP provided information to Czech spies for a decade, including details about colleagues in Parliament.
Mawby was an unusual Tory - a working class trade unionist who had been selected as MP for Totnes, Devon, in 1955.
But just how unusual he was is only now clear.
It was previously known that some Labour MPs - including one minister, John Stonehouse - had spied for the Czechs, but this is the first time a Conservative has been shown to have engaged in passing information to communist spies.
According to his Czech file - which runs for hundreds of pages - Mawby, codename Laval, was first contacted when he attended a cocktail party in the Czechoslovak embassy in November 1960. At first there were just occasional social contacts between him and the Czechs, but they became more regular through 1961.
The central weakness which the Czech spies exploited was money.
"His leisure time he spends in bars… and also loves gambling," one noted. "While playing roulette and other games he is willing to accept a monetary 'loan' which was exploited twice."
The Czechs played it slow - reeling the MP in with conversations about politics and trade unions. They noted that this talk was "of only limited interest", but then "to improve the quality of his information, we focused Mawby on supplying documents from Parliament", a spy wrote in a report.
In time Mawby would be paid directly for political information - normally £100 per time. One file in the archives has a receipt signed by Mawby himself for that sum.
Mawby later handed over lists of parliamentary committees, details of fellow Tories and a supposedly confidential parliamentary investigation into a Conservative peer.
"Mawby has also promised to carry out tasks such as asking questions in Parliament according to our needs," the Czech handler wrote in a plan on how to use him in 1962.
That plan involved asking him to supply more confidential material from Parliament "gradually deepening the compromising of his position".
Mawby did not have access to any top secret information - like other MPs, what he provided the Czechs was often political gossip, for instance, reporting on leadership machinations within the Tory party in a handwritten 1963 note.
The Czechs seem to have been surprised - even worried - when he became assistant postmaster general and a junior minister in 1963.
His handler notes with some concern that the ministerial appointment means that Mawby's salary will go up by £2,000 a year "and my whole project of using him has been undermined".
Yet Mawby continued his meetings even after he was promoted.
As the 1964 election approached, the Czechs realised he could lose his ministerial job and developed a plan of paying him roughly £400 a year for information in order to make up for the loss of salary.
At a meeting on 11 November at the 500 Club, the relationship was put onto a formal footing with an agreed set of tasks in return for the money which would be handed over in a "conspiring way" such as in a taxi.
The flow of information continued. At the end of a meeting at L'Apertif Grill in November 1965, Mawby appears to have handed over a piece of paper with the names of three new officials in the Conservative Party with the request to gather more details.
"Laval fulfilled this task before our next meeting," the Czech spy writes. He also asked Mawby for information about the prime minister's office in the House of Commons and the layout of the room.
"I recommend giving Laval at the following meeting before Christmas 100 pounds on the understanding that he will provide us with the requested plan," the Czech handler wrote to Prague, also mentioning that Mawby appeared uninterested in why the material was wanted.
Mawby did indeed go on to provide official and handwritten floorplans - also passing on details of who provided security at the office.
How the Czechs intended to use this information is unclear - although it was possible it could have been to break in or to plant bugs.
There is no indication in MI5's authorised history of the Security Service having known of Mawby's activities and a Whitehall official declined to comment.
Meetings took place sometimes three or four times in a month but dwindled by the end of the 60s. The file shows the relationship ending in November 1971.
Two months earlier the British government had kicked out more than 100 Soviet diplomats from London.
A note in Mawby's file reads: "Considering the worsening operational conditions in Great Britain and after evaluating dangerous signals… we are forbidding all contacts with him."
Mawby was deselected as an MP in 1983 and died in 1990.