Baroness Shirley Williams, one of the most senior figures in the Liberal Democrats, says she will support an amendment by equal marriage opponent Lord Hylton.

Lord Hylton has suggested that the word “union” is strong enough to describe the bond between same-sex couples.

For page 1 of the Marriage (Same Sex Couples) Bill his amendment states: “leave out ‘Marriage’ and insert ‘Union’”.

Peers resumed debate of the bill on Monday afternoon.

Baroness Williams argued that, as well as biological differences, men and women approach relationships differently – making opposite-sex couples the most “stable” parents.

She said: “Equality is not the same as sameness”.

Originally a Labour Member of Parliament and Education Secretary under former Prime Minister Jim Callaghan, she was one of the “Gang of Four” rebels who founded the Social Democratic Party (SDP) in 1981.

The party merged with the Liberal Party to form the Liberal Democrats in 1988.

From 2001–2004, Baroness served as Leader of the Liberal Democrats in the House of Lords.

In this month’s second reading vote of the bill, Lord Hylton, a crossbench hereditary peer, said: “I regret very much that the fine old English and French word ‘gay’ has, in my lifetime, been appropriated by a small but vocal minority of the population.”

“The result is that it can no longer be used in its original and rather delightful meaning.”

He added: “Now, under the pretext of securing equality, Her Majesty’s Government are proposing to change the meaning of marriage.”