According to Sky sources, the Democratic Unionist Party is planning to vote with Labour during a House of Commons debate on NHS pay on Wednesday afternoon.
Although the result of the vote will be non-binding on ministers, it will heap pressure on the Prime Minister to make a bigger offer to public sector workers later this year.
On Tuesday, Downing Street confirmed the 1% limit on public sector pay rises is to be scrapped.
Prisons officers and police will enjoy raises above 1% for the first time since 2010, with Theresa May ready to show “flexibility” on pay for other public sector workers in 2018/19, according to Number 10.
But the move, announced on the same day figures showed the inflation rate has reached 2.9%, did little to ease the criticism of the Government’s pay restraints.
Union leaders raised the possibility of illegal strikes while demanding an immediate 5% pay boost for all public sector workers.
Police bosses also said their rise did not go far enough.
Wednesday’s opposition day debate on NHS pay demands the Government “end the public sector pay cap in the NHS and give NHS workers a fair pay rise”.
At the beginning of proceedings in the House of Commons, DUP MP Ian Paisley signalled his party’s support for Labour’s motion.
He said: “I must say that myself and my colleagues are minded to support the motion… put before the House this evening.”