PM '˜not burdened' with name of agent

In 1964 Sir Anthony Blunt, the surveyor of the Queen’s pictures, sensationally confessed that he was the so-called “fourth man” in the Cambridge spy ring, which also included Kim Philby, Guy Burgess and Donald Maclean.

He admitted – following an offer of immunity from prosecution – passing thousands of documents to the Russian KGB while working as an officer for MI5 during the Second World War.

But when his treachery was finally made public in 1979, Margaret Thatcher told an astonished House of Commons that Sir Alec had not been informed of Blunt’s admission even though he was Prime Minister at the time.

The then Home Secretary, Henry Brooke, took it upon himself not to tell Sir Alec – a decision he admitted may well have been a mistake.

In a note to Mrs Thatcher, he wrote: “I have written to Alec to explain why in April 1964 I did not bring him in on what was happening about Blunt and to say how sorry I am if in my well-meant effort not to add to his burden I may, with hindsight, have exercised by discretion wrongly.”

The files also show how, following Blunt’s death in 1983, Mrs Thatcher rejected an offer to settle his estate duties through the donation of an old master, bought by Blunt in the 1930s for just £100, and now valued at £350,000.

Mrs Thatcher was initially enthusiastic when his estate offered it to the Government at the knock-down price of £190,000.

But she changed her mind when she learned it would mean the taxman losing £200,000 in duties with Blunt’s estate benefiting by £40,000.