Long-held suspicions that Margaret Thatcher lied to the House of Commons over the Westland helicopter affair in 1986 have been endorsed by her official biographer, Charles Moore.

Asked if Mrs Thatcher had deliberately misled Parliament over her knowledge of a leaked letter damaging to the then Defence Secretary, her arch political rival Michael Heseltine, Mr Moore replied: “I think ‘yes’ is the short answer.”

In an interview about What She Wants, the second volume of his authorised biography of the late Prime Minister, Mr Moore said Mrs Thatcher’s hands “were not entirely clean” in respect of the leaking of the letter. At the time, Mrs Thatcher was adamant that she was completely unaware that a senior Cabinet member had asked Colette Bowe, an official at the Department of Trade and Industry, to leak the letter.

In a speech to the House of Commons on 23 January 1986, Mrs Thatcher said that although it was right that the contents of the letter had been made public, “had I been consulted [about the leaking of the letter], I should have said that a different way must be found of making the relevant facts known”.

But Mr Moore, who persuaded Ms Bowe to break her silence about the incident, believes the truth is more complicated. He says Ms Bowe told him she couldn’t understand why the Government had commissioned an inquiry into the leak “because Number 10 knew perfectly well why it had been leaked”.

“What Colette Bowe actually had as all this was coming was a direct message brought to her from Number 10 – before the inquiry but after the leak – giving Number 10’s approval,” Mr Moore said in his conversation with the Chancellor, George Osborne, organised by the Policy Exchange think-tank. “I’m not saying Mrs Thatcher knew exactly how this was done, but the point is – in the words of the great Charles Powell [Mrs Thatcher’s private secretary] himself – her hands were not entirely clean. This is in a sense a minor thing, but in the incredible tension of this moment I think it would have probably had her out.”

Michael Heseltine quit three days after the letter became public (Getty)

If she had resigned, Mr Osborne observed, “that would have changed this entire part of British history”.

Mr Moore added: “Her reputation was for this kind of honesty … and it [the leaking of the letter] looked shabby. And obviously people like Michael Heseltine, and less obviously but truly Geoffrey Howe, wanted to succeed her, which she well knew, and it was a very dangerous moment.”

In the event, it was Mr Heseltine – at loggerheads with Mrs Thatcher over the best way to secure the survival of Westland, Britain’s last helicopter manufacturer – who became the first major casualty of the affair. Three days after the letter was leaked he stormed out of a heated Cabinet meeting about the company and announced his resignation.

Under pressure from the attorney-general, Mrs Thatcher commissioned an inquiry into the leak chaired by the Cabinet Secretary, Sir Robert Armstrong. His report, delivered on 21 January 1986 concluded that Leon Brittan had asked Ms Bowe – via an intermediary – to leak the letter, but Mrs Thatcher insisted she was unaware of his involvement.

Lord Kinnock, who as leader of the Opposition in 1986 led the attack on Mrs Thatcher over the leak, said “the whole affair would have been entirely different” if Ms Bowe’s revelation had been known at the time.

“Even if she had said it to the House of Commons select committee that interviewed her later, that would have put an entirely different dimension on the whole issue,” he said. “At the time my bones told me that [Mrs Thatcher’s involvement] was the only way in which such a letter could have been leaked and probably composed. I asked all those questions in the debates of course, and of course the answer I got amounted to what Charles Moore describes as ‘misleading the House’. All the time I said to the people who I was working with, ‘I’m certain of what happened but I can’t damn well prove it.’”

Sir Bernard Ingham, Mrs Thatcher’s former press secretary, said the idea that she was aware the letter was being leaked was “bunkum”. “I have no evidence she wanted it leaked, indeed leaking was anathema to her,” he said. “We knew who leaked it – it was the DTI. Is that consistent with Mrs Thatcher wanting to leak it? Of course it isn’t.

Margaret Thatcher: Life in pictures Show all 35 1 /35 Margaret Thatcher: Life in pictures Margaret Thatcher: Life in pictures Margaret Thatcher as prime minister, 1 August 1980 Getty Images Margaret Thatcher: Life in pictures 1950. British prime minister Margaret Thatcher, then Miss Margaret Roberts, attending a garden party at Buckingham Palace, as a Conservative MP Margaret Thatcher: Life in pictures Margaret Roberts, later Thatcher, the youngest candidate in the Conservative Party, plans her election campaign Margaret Thatcher: Life in pictures Future British prime minister, Margaret Thatcher with her parents and sister, Muriel in 1954. Her father Alfred Roberts was an Alderman and served as the Mayor of Grantham from November 1945 to 1946 Margaret Thatcher: Life in pictures Party leader Margaret Thatcher presents the Tory policy leaflet 'The Right Approach' at the Conservative Central Office, 1976 Margaret Thatcher: Life in pictures Margaret Thatcher next to a campaign poster on becoming the first woman to hold the office of Prime Minister, 4 May 1979 Margaret Thatcher: Life in pictures Margaret Thatcher, accompanied by her husband Denis (1915 - 2003) and children Carol and Mark, on election night when she led the Conservative Party to victory in the British General Election, 1979 Margaret Thatcher: Life in pictures British conservative politician, Margaret Thatcher, with William Whitelaw and Peter Kirk at a referendum conference on Europe, 3 June 1975 Margaret Thatcher: Life in pictures Prime Minister Thatcher sits with her new Cabinet at No 10 Downing Street Getty Images Margaret Thatcher: Life in pictures On becoming Prime Minister in May1979, Margaret Thatcher sought ways to cut spending Getty Images Margaret Thatcher: Life in pictures Fleet Street printers on a protest march against the Prime Minister, Margaret Thatcher, March 1980 Margaret Thatcher: Life in pictures Emlyn Hughes (R) and Kevin Keegan (L) enjoy a joke with Mrs Margaret Thatcher outside No 10 Downing Street in 1980 Getty Images Margaret Thatcher: Life in pictures Margaret Thatcher with Chairman of the Conservative Party Lord Peter Thorneycroft (1909 - 1994, left) at the party conference in Brighton, 1980 Keystone/Hulton Archive/Getty Images Margaret Thatcher: Life in pictures British Conservative Prime Minister, Margaret Thatcher, with Geoffrey Howe, Keith Joseph, John Nott, Norman Tebbit on the Conservative front bench in the House of Commons, 4 November 1981 Central Press/Getty Images Margaret Thatcher: Life in pictures British prime minister Margaret Thatcher, American president Ronald Reagan (left) and US Secretary of State Alexander Haig (centre) outside Number 10, Downing Street during Reagan's state visit to London. The new UK/USA 'Special Relationship' was founded on the rapport between the two leaders, 1982 Margaret Thatcher: Life in pictures British Conservative Prime Minister Margaret Thatcher with Indian premier Indira Gandhi (1917 - 1984), outside 10 Downing Street, 22 March 1982 Central Press/Getty Images Margaret Thatcher: Life in pictures British Prime Minister, Margaret Thatcher, with husband Denis (1915 - 2003) and children Mark and Carol, 14 January 1982 Margaret Thatcher: Life in pictures British Prime Minister Margaret Thatcher shares a joke with American President Ronald Reagan, at No. 10 Downing Street, London, 1984 Getty Images Margaret Thatcher: Life in pictures The devastation caused by an IRA bomb at the Grand Hotel in Brighton, where British prime minister Margaret Thatcher and her ministers were staying for the 1984 Conservative Party Conference, 12 October 1984 David Wimsett/Getty Images Margaret Thatcher: Life in pictures British prime minister Margaret Thatcher holding a chimpanzee, 1 January 1985 Keystone/Getty Images Margaret Thatcher: Life in pictures British Prime Minister Margaret Thatcher dances with US President Ronald Reagan 16 November 1988 following a state dinner given in her honor at the White House Getty Images Margaret Thatcher: Life in pictures Four of the voice artists on the satirical TV programme, 'Spitting Image' with some of the puppets, for which they provide voices (Margaret Thatcher, Robin Day, Vincent Price and Prince Philip). Back, left to right: Chris Barrie, Jon Glover, Enn Reitel. Front: Steve Nallon, 1 February 1985 Express/Express/Getty Images Margaret Thatcher: Life in pictures Margaret Thatcher looks pensive at the Conservative Party Conference, October 1985 Getty Images Margaret Thatcher: Life in pictures From left to right, French president Francois Mitterrand (1916 - 1996), British prime minister Margaret Thatcher and foreign secretary Geoffrey Howe at the signing of the 'Fixed Channel Link' treaty, 1986 Margaret Thatcher: Life in pictures Margaret Thatcher celebrates on election night in 1987 Margaret Thatcher: Life in pictures British Prime Minister Margaret Thatcher flanked by her husband Denis, addresses the press, 28 November 1990 for the last time in front of 10 Downing Street in London prior to hand her resignation as prime minister to Queen Elizabeth II Margaret Thatcher: Life in pictures Margaret Thatcher, then prime minister, famously posed at the wheel of a Nissan Bluebird PA Margaret Thatcher: Life in pictures Out of action: when the miners' strikes failed to budge Margaret Thatcher, it spelt the end of union power PA Margaret Thatcher: Life in pictures On the afternoon of Saturday 31 March, a peaceful protest against Margaret Thatcher's unpopular poll tax turned to violence in the centre of London, 1 April 1990 Jon Jones Margaret Thatcher: Life in pictures Baroness Margaret Thatcher in 2006 Chris Jackson/Getty Margaret Thatcher: Life in pictures Baroness Margaret Thatcher and David Cameron photographed together in 2007 Margaret Thatcher: Life in pictures Carol Thatcher and her mother Margaret, the former prime minister, in 2008 REUTERS Margaret Thatcher: Life in pictures Former Prime Ministers Baroness Thatcher and Tony Blair stand beside current Prime Minister Gordon Brown AP Margaret Thatcher: Life in pictures Margaret Thatcher waves to the press at her home, after leaving Cromwell Hospital on 1 November 2010 Margaret Thatcher: Life in pictures Prime Minister Baroness Margaret Thatcher meets with current British Prime Minister David Cameron, inside 10 Downing Street in London, 8 June 2010 Getty Images

“I thought the letter was a stupid thing to do in any case – why not simply write to people and tell them they’d done the wrong thing?”

Sir Bernard confirmed the fevered atmosphere at the time, saying “the Government was falling apart and Heseltine was consumed with frustrated ambition”. He said: “Mrs Thatcher never gave me to understand that she knew what was going on and her parliamentary private secretary didn’t know either.”

He added: “I’m getting a bit fed up of this effort to implicate me in some sort of conspiracy.”

Sir Robert, now Lord Armstrong, told The IoS: “One couldn’t prove anything, but I didn’t have a positive reason to think she knew.”

And if he had? “There would have been more explaining to do, wouldn’t there. I rather doubt she would have had to resign although it was a fevered atmosphere.”