Liberal Democrat peer Baroness Williams has tabled a new amendment urging for a legal separation between same-sex marriages and opposite sex marriages.

It states: “marriage between same-sex couples may be defined as same-sex marriage, lawful marriages between same-sex couples may be defined as same-sex marriages, lawful marriage between a man and a woman may be defined as opposite sex marriage and lawful marriages between a man and a woman may be defined as opposite sex marriages.”

Baroness Williams, one of the most senior figures in the Liberal Democrats, has also tabled an amendment allowing registrars to opt out of providing marriages to same-sex couples.

Report stage of the Marriage (Same Sex Couples) Bill, where the latest proposed amendments will be debated, commences on Monday.

Last month Baroness Williams supported an unsuccessful amendment by equal marriage opponent Lord Hylton.

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

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”.

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