The Duke, Earl Marshal of England, a leading courtier to the Queen, and occupant of Arundel Castle, in Sussex, will be joined by the Bishop of Chester, Lord Longford and the former Chief of the Defence Staff, Lord Hill-Norton, in seeking to overturn the reduction to 18 agreed by the Commons in February. He said last week that he thought his move would be 'carried without question'.
Describing his amendment as 'a way to reduce the incidence of Aids,' the Duke, 78, said: 'Homosexual lifestyle reduces life expectancy from 75 to 42. As a Christian, I believe that sodomy is wrong. If you legalise it, it will lead to them doing more of it.'
Yesterday Lord Hill-Norton refused to commment.
Angela Mason, executive director of Stonewall, the gay rights lobby, said that any reversal of the Commons' vote would be a 'constitutional travesty'. She promised: 'If it comes, there will be a renewed campaign of very great vigour.' Another gay lobbyist said he was '60 per cent sure that 21 will happen'.
Widely differing predictions of the outcome of today's debate reflect the difficulty of forecasting the vote. As in the Commons, there will be no whipping on the issue, which is expected to divide members of all parties. Much would depend, senior sources said, on exactly who turned up - and on how persuasive the speeches of each side were.
The duke's amendment is one of a complex series tabled to Clause 139 of the Criminal Justice Bill. Another, from the Labour frontbencher Lord McIntosh, which reduces the age of consent to 16, is thought likely to fail.
Gay activists are, however, hopeful of success for moves by Lord Ponsonby and Viscount Falkland to ensure that under-age boys involved with men over the limit are not themselves prosecuted. Another amendment - to equalise penalties for male and female rape - is also thought likely to succeed.
Defeat for the Government's preferred option of 18 would gravely embarrass ministers and seriously hamper their defence in the European Court of Human Rights against three actions brought by gay teenagers. Stonewall will announce today that a fourth case, involving a 17-year-old, is to be launched.
Although the Commons would be able - and willing - to reverse any Lords defeat, the time required could throw government business off course and delay the start of MPs' summer holidays.
There were hints last week that the Government might be preparing concessions to head off defeat on the age of consent, with some mentioning the possibility of a retreat on the previously-announced commitment to decriminalise homosexuality in the armed forces.
The fight to lower the age of homosexual consent to 16 was one of the longest and most important campaigns by a lobby group in British politics, and was the first time since the late 1960s that homosexuality was at the forefront of public debate. Sir Ian McKellen, Stephen Fry, Julian Clary and other high-profile homosexuals were all prominent in urging change.