The former Conservative cabinet minister Lord Gilmour has died aged 81.
Lord Gilmour was a critic of the government's economic policy
His eldest son David Gilmour said he had died at London's West Middlesex Hospital following a short illness.
Ian Gilmour served as defence secretary during Edward Heath's administration, before becoming Lord Privy Seal in Margaret Thatcher's first government.
In September 1981 he was sacked by Mrs Thatcher and remained a prominent critic of what he regarded as extreme Thatcherite policies.
He went on to the back benches where he remained until he went to the House of Lords as Lord Gilmour of Craigmillar in 1992.
Pro-Europe, against hanging, opposed to the poll tax and unhappy with monetarist economics, Lord Gilmour was a leading Conservative "wet" who became a persistent and outspoken critic of Thatcherism.
He responded to his sacking by Mrs Thatcher by issuing a statement declaring that she was steering "full speed ahead for the rocks".
Douglas Hurd, who later became foreign secretary, observed: "One wonders how he was invited to serve in her Cabinet, and why he agreed."
Ian Hedworth John Little Gilmour was born on 8 July 1926 and was educated at Eton and Balliol College, Oxford.
Wealthy and well connected, he was editor and proprietor of the Spectator magazine for a time in the 1950s, when he backed homosexual law reform and called the then prime minister, Sir Anthony Eden, a "liar" over the Suez invasion.
He went on to serve in the House of Commons as Conservative MP for Norfolk Central from 1962 to 1974 and Chesham and Amersham from 1974 until his retirement in 1992.
He became Army Under-Secretary in 1970 and Defence Procurement Minister the following year. He later entered the Cabinet as Defence Secretary.
After Margaret Thatcher's election as Conservative leader, he warned that a shift to the right could cost the party the next general election.
After his appointment as Lord Privy Seal in her government, Lord Gilmour continued to criticise Mrs Thatcher's policies, opposing monetarism and the dismantling of the welfare state.
Mrs Thatcher was left with no alternative but to sack him in 1981.
From the back benches, Lord Gilmour continued to rebel against Thatcherism, opposing the abolition of the Greater London Council, the poll tax and dental and eye test charges.
He was once quoted as saying: "Socialists may look forward to some grim Utopia. Conservatives have no such illusions about the future or the past.
"For them there has never been a 'golden age' and there never will be. Similarly there is no fixed or golden policy to which the Conservative Party could or should turn."
Lord Gilmour retired from the Commons in 1992 and was given a peerage.
The leader of the Conservatives in the House of Lords, Lord Strathclyde said: "Sir Ian Gilmour had a lifetime of service to the public and the Conservative Party.
"He will be greatly missed by his friends from all sides of the political spectrum."
He leaves four sons and a daughter.