Senior Whitehall officials privately admitted to extraordinary security lapses over two of Britain’s most notorious spies for the Soviet Union, and then conspired to play down the hugely embarrassing scandal of their flight to Moscow, according to top secret documents seen by the Guardian.

A hitherto unpublished report on the flight of Donald Maclean and Guy Burgess – two prominent members of the Cambridge spy ring – more than 60 years ago, says they could have been suspected sooner had the Foreign Office linked their bouts of extreme drunken behaviour to their spying.

For years, Foreign Office, MI5, and MI6, officials failed to notice the continuing communist sympathies of the two men even though they did little to hide them.

One document reveals that the head of security at the Foreign Office, George Carey-Foster, told senior MI5 and MI6 officers that details about an internal inquiry into Burgess and Maclean should not be passed to the Ministry of Defence or to Whitehall’s joint intelligence committee.

In a determined attempt to cover up the damage the two defectors caused, Carey-Foster added that senior British intelligence and defence officials should tell the US they could give them “no confidential information” about the two spies.

The extent to which the Foreign Office attempted to cover-up one of Britain’s most serious cold war spy scandals is revealed in papers released following a freedom of information act request by Stewart Purvis and Jeff Hulbert, authors of Guy Burgess: The Spy Who Knew Everyone, to be published by Biteback early next year.

The documents are among nearly 200 MI5, Foreign Office and Cabinet Office files to be released at the National Archives next week.

Facebook Twitter Pinterest Donald Maclean. Photograph: Keystone/Getty Images

An official inquiry into Burgess and Maclean by Sir Alexander Cadogan, former top official at the FCO, concluded there was a serious lack of communication between the FCO’s personnel and security departments. As the inquiry put it, there was “an artificial separation between political reliability and personal reliability of members of the service”.

Dick White, the head of MI5, told the inquiry that Burgess’s “weakness, including his indiscretion and his homosexual tendencies were well known in MI5 but they had not regarded him as a member of the Communist party or as a possible Soviet agent since they did not think him capable of sustaining such a role”.

According to the hitherto secret Cadogan report, White said the 1930s “was a time dangerous for young minds of an intellectual bent. The disillusionment of young men at that period had led them to find an outlet for their idealism in the Communist party.” White added: “This was particularly dangerous for the civil service, which of course drew its recruits precisely from the more intelligence university-trained men and women. Once a Communist or Russian agent had got into a government department it was all too easy for him to remain there undetected.”

The inquiry also showed that the failure to act sooner against the two men was a result of tolerance as much as complacency. “It was no part of our duty to weigh the moral issues raised by homosexuality”, Cadogan said. He added in a covering letter to the FCO, marked “personal and secret”: “It would be repugnant to our national and service traditions and damaging to the mutual confidence among colleagues which is the great strength of the foreign service, if tale-bearing should be thought to be encouraged and if spying and delation were suspected to be part of the regularly employed equipment of the authorities.”

The documents include a long list of top secret papers which Maclean had access to. Among them was the FO’s “warbook”, a record of Churchill’s recent visit to the US, and joint intelligence committee and Nato papers.

Burgess and Maclean disappeared on 25 May 1951. News of their flight broke on 7 June. The pair emerged publicly in Moscow more than five years later, in February 1956.