Mr Greer, whose lobbying firm collapsed after being plunged into the "cash-for-questions" scandal, said many gay MPs were married but preferred to hide their sexuality.
His comments, on Radio 5 Live's gay show Out This Week, will have been noted with concern in government where he is regarded as a loose cannon after the fall-out from the Neil Hamilton libel action which was disastrous to his business.
Mr Greer is at present promoting his autobiography, One Man's Word, which includes a number of damaging disclosures about the Government. The lobbyist formerly worked closely with the Conservative party at the highest levels.
His own homosexuality is an open secret at Westminster and many politicians accepted the lobbyist and Clive, his partner of 13 years, as a couple and even dined with them at their home, he said last night.
Mr Greer's connections and his knowledge of the gay scene put him in a position to make revelations potentially damaging to the Government so close to the election, when it will be attempting to underline its commitment to family values.
Last night on Radio Five, Mr Greer said it filled him with "sadness" that the MPs were not prepared to be open about their sexuality. "Some are married but are still gay," he said. "Many are actively gay, others perhaps are not married but have chosen not to make any declaration as to their sexuality. I think it is very sad if you are a Member of Parliament and you are gay, if you feel that you have to get married to cover that sexuality [in pursuit of] an upwardly mobile career to the Cabinet."
Mr Greer's own clients were only likely to be worried if he flaunted his homosexuality in public and went on gay pride marches, he said. Although OutRage, the gay lobbying group, has threatened to "out" gay MPs who are hypocritical about their views on family values, Mr Greer has not said he will identify any of the gay MPs he knows.
But he said last night: "There is always a lot of gossip at Westminster" and many gay people were "often indiscreet in their conversations".
As the head of Ian Greer Associates, the lobbyist was unrivalled in his ability to bring ministers and businessman together. The IGA reception was the hot ticket at Conservative party conferences and the offices in Catherine Place played host to three lavish lunches a day for selected MPs and business clients.
At the IGA 10th anniversary party, Mr Greer is said to have sent a waiter across town to serve a gin and tonic to the Prime Minister, John Major.
He has also been a close friend of a number of Tory ministers and worked on the fringes of John Major's leadership campaign.
The relationship has soured since his pounds 4m company went into liquidation and his client list evaporated following the collapse of the cash-for- questions libel action. In Mr Greer's autobiography, he claimed that he warned senior Conservatives in the late Eighties of a serious problem with Tory MPs asking for free flights.
He said he was asked to raise the matter by Lord King of Wartnaby, the former chairman of British Airways, who became horrified at the number of free flights and upgrades being sought by Tory MPs for themselves and their families.
The problem began when Lord King gave them to supporters of the controversial legislation to privatise the airline. Mr Greer claimed that Conservative MPs became so accustomed to the practice that they would not stop.
The disclosures have already threatened to reignite the row over political sleaze. Mr Greer, who declares himself to be virtually bankrupt is unlikely to shirk at further revelations to boost the book's promotion.Reuse content