An amendment to legislation to be debated on Tuesday would force the legalisation of equal marriage in Northern Ireland, the last part of the British Isles where it is prohibited.

The Northern Ireland (Executive Formation) Bill extends the deadline for talks to reform the power-sharing executive, which has been suspended since January 2017 due to the ongoing political crisis involving Sinn Fein and the DUP.

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Speaking for the SNP during the second reading of the bill on Monday evening, nationalist MP Gavin Newlands sparked anger by the Commons that his group would abstain out of respect for the devolution settlement.

“On the various amendments tabled by the Labour Front-Bench team and others for Committee tomorrow, it is a long-held principle that we on the SNP Benches do not vote on matters devolved to other parts of the UK that solely affect that country,” he said.

“We are not blind to the circumstances in Northern Ireland, but we intend to stick to that principle.”

But after an outcry on social media and a fierce lobbying effort by a handful of SNP MPs, the party’s Westminster group leader Ian Blackford signaled that nationalists would be given a free vote, tweeting: “We believe it is right to give MPs a vote on these important human rights issues. If the amendments are called to a vote, I will be voting in favour.”

SNP MPs including Joanna Cherry, Stewart McDonald and Hannah Bardell welcomed the u-turn and said they would vote for the amendments.

There had been significant anger within the SNP group, with MPs “trying to make others realise how badly this will go down”.

One SNP source claimed the issue hadn’t been discussed by the SNP group before Mr Newlands spoke in the Commons, and said the decision to abstain wasn’t supported by the First Minister. “God knows how we got into this position,” the source said.

Nicola Sturgeon tweeted minutes after Mr Blackford, saying that “I’m not in the House of Commons but if I was, I’d vote for amendments on [Northern Ireland] equal marriage and women’s right to choose”.

Another party source highlighted a column in Mr Blackford’s name on Sunday, supporting trans equality amid a bitter row over the issue of gender self-identification, saying: “If you're going to put out puff pieces one day, it's ludicrous to say you're not going to vote on the substance the next day.”

Mr Blackford’s full statement posted on twitter said: “For those asking, I strongly support equal marriage, and women’s right to choose on abortion.

“While the SNP will always defend the principle of devolution, there are a specific set of circumstances in Northern Ireland where there has been no functioning Assembly for an extended period.

“In these circumstances, we believe it is right to give MPs a vote on these important human rights issues. If the amendments are called to a vote, I will be voting in favour.”

An SNP spokesman said the decision was “not a u-turn” because the position on how to vote would not have been finalised until today.

SNP forced into u-turn on equal marriage in Northern Ireland