The half-way measure, tabled by four Conservatives, was condemned by Jerry Hayes, Tory MP for Harlow, a supporter of a common age of 16 for gays and heterosexuals. He said: 'I fail to understand how one can compromise on equality. People are either equal or they are not.'
The amendment has been tabled in the names of Sir Anthony Durant (Reading), an archetypal knight of the shires, Tim Devlin (Stockton), Richard Page (Hertfordshire South West) and Spencer Batiste (Elmet) as an alternative to Edwina Currie's amendment to equalise the age.
Amid reports of a dearth of volunteers willing to put their names to the 18 compromise, accounting for its delay, there was growing confidence yesterday that Mrs Currie's amendment will scrape a majority.
One MP said of yesterday's amendment: 'This is a put-up job. MPs have been reluctant to put forward their names because the amendment is a compromise.'
In the free vote later this month the 16 option is expected to be taken first. That could increase its chances because some MPs will support it rather than risk the possibility of the 18 compromise failing to secure a majority in favour.
Equally, however, some MPs who are lukewarm about change will be content to be recorded as having voted for the compromise regardless of whether it can secure a majority.
Mr Devlin yesterday rejected the argument of Sir Ian McKellen, the actor, and gay rights organisations that a vote for 18 would be unprincipled. The proposal did not accept the argument for equality and was based on removing anomalies.
'The amendment removes the anomalies that at the age of 18 you can fight for your country, vote and get married without consent but cannot indulge in a homosexual act with another male.'
Most government ministers will support the 18 compromise favoured by Michael Howard, the Home Secretary and John Major. But virtually all the Labour Party, most of the Liberal Democrats and a sizeable minority of Tories support a common age.