Gay marriage is set to become law after the House of Lords gave its assent in the third reading stage of the Government's Marriage (Same-Sex Couples) Bill. 

Amendments will be debated by MPs on Tuesday night but the substance of the Bill, ending the centuries-old definition of marriage as a union between a man and a woman, will be largely unaltered.  

The Queen is expected to give the Bill Royal Assent before the end of the week.  

Pro-gay marriage supporters were celebrating outside Parliament on Monday night with placards saying 'Thank you for supporting equal marriage'.

However, campaign group the Coalition for Marriage said "wrecking marriage" would "come back to bite" the Prime Minister.  

The coalition, whose petition in support of traditional marriage amassed over 700,000 signatures, criticised the Government's failure to pass suitable safeguards for people who believe in traditional marriage, saying it was "inevitable" that there would be cases of people being penalised for their beliefs once gay marriage becomes law.

Colin Hart, coalition director, said the group would continue to campaign for additional safeguards and measures to protect people who believe in traditional marriage, including teachers and public sector chaplains.

The coalition wants to see measures to protect 'reasonable accommodation' of beliefs in the workplace and freedom of speech, including the right to publicly support traditional marriage without fear of being penalised under the law.  

"There was a huge amount of political arm-twisting against MPs and Peers. Whatever the parties may say about so-called free votes, these votes were not genuinely free," he said.

"Mr Cameron needs to remember that the Coalition for Marriage has nearly 700,000 supporters, nearly six times the number of members of the Conservative Party.

"They are just ordinary men and women, not part of the ruling elite. They are passionate, motivated and determined to fight on against a law that renders terms like husband and wife meaningless and threatens one of the foundations of the institution of marriage: fidelity and faithfulness.

"These concepts may not matter to the leaders of the three main political parties, who are drawn from a very narrow liberal political class, but they do matter to people up and down the country who believe that marriage is special, unique and the bedrock of stable families."