The British parliament's decision to bypass politicians in Northern Ireland and legislate on same-sex marriage could deepen a stalemate in the region at the heart of Britain's struggle to leave the European Union, a senior lawmaker warned on Thursday.

British lawmakers voted overwhelmingly on Tuesday to compel the government to legalise same-sex marriage and also extend abortion rights in Northern Ireland if the local assembly, which collapsed in 2017, has not resumed by Oct. 21.

The surprise intervention could prolong a political standoff in the region most exposed by Britain's decision to leave the EU - and whose future has become the main sticking point for an exit deal, pushing London towards a no-deal exit.

Irish nationalists Sinn Fein pulled out of the power-sharing executive two-and-a-half years ago arguing that their pro-British rivals in government were not treating them as partners.

On-off talks to restore it resumed in May after a hiatus of more than a year but have made little progress. The pro-British Democratic Unionist Party (DUP) has repeatedly refused to accept a series of preconditions set down by Sinn Fein.

But as one of those demands is that same-sex marriage be legalised, Sinn Fein is now likely to stand back and let the British government legislate instead, a senior DUP politician said.

"I think this undermines the prospect of getting devolution restored because there is little incentive for Sinn Fein ... It makes it more difficult now to get devolution restored," Jeffrey Donaldson told BBC Radio on Wednesday.

Previous attempts to legislate for same-sex marriage have been blocked by the socially conservative DUP, despite polls in recent years showing most in the region are in favour.

Even though it props up the Conservative Party-led minority government in London, Donaldson said there was nothing the DUP could do to stop the legislation.