Veteran gay rights activist Peter Tatchell wants supporters to lobby the House of Lords warning they may veto same-sex marriage bill for England and Wales.

The upper house of the UK parliament will likely debate the Marriage (Same-Sex Couples) Bill next month, although dates have not yet been set.

But while it is possible the Lords may amend the bill’s details, other experts believe the chances of a veto are tiny.

And even if they do overturn the bill, Conservative Prime Minister David Cameron appears to be sufficiently determined to pass the legislation that he would almost certainly invoke the Parliament Act. That allows bills that pass through the elected House of Commons to be forced through the unelected Lords.

Tatchell himself, in a statement released today, said: ‘While it would be unusual and unethical for the unelected Lords to overturn a bill passed by a huge 225-vote majority in the Commons, it is not impossible or unlawful.

‘In the event of defeat in the Lords, it is not clear whether the government would invoke the Parliament Act to secure the bill’s passage.’

Tatchell, who has been a noted advocate of marriage equality and is a coordinator of the Equal Love campaign, added: ‘The biggest uncertainty facing the same-sex marriage bill is how the House of Lords will vote.

‘We just don’t know how much opposition there will be in the upper house. It would be a big mistake to assume that equal marriage is a done deal.

‘There is no room for complacency. We need to lobby the Lords just like we lobbied the Commons. Church leaders and right-wing peers are determined to derail the Marriage (Same Sex Couples) Bill. They’ll try to vote it down and introduce wrecking amendments.

‘Last month, Lord Dear predicted that the bill could be defeated by peers.’

Likely amendments include giving an opt-out to registrars who don’t want to marry lesbian or gay couples.

In practice, lobbying long-term supporters of equality in the House of Lords may be pointless but letters may help for peers who are still undecided on the issue.

But Tatchell wants people to counter-balance the pressure coming from religious extremists who are trying to scupper the bill, despite it being supported by a clear majority of the British public.

Lord Deer has emerged as a possible leader of a Conservative rebellion in the Lords against marriage equality. But many of the 213 Conservative peers would still support Cameron’s Conservative-Liberal Democrat government on the bill.

There are 220 Labour and 90 Liberal Democrat peers – and both those parties also support marriage equality. Of the rest of the 760-strong house, the main group is the 178 independent Crossbenchers. There are also 25 bishops, whose Church of England opposes gay marriage, and 32 others, including a handful from smaller parties.

Separate equal marriage legislation is currently being passed through the Scottish Parliament.