According to Sky sources, the Democratic Unionist Party (DUP) are 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.
Prison 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 had 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.
Both the Police Federation of England and Wales and the Prison Officers Association said the rises 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”.
DUP MPs signed a similar early day motion earlier this year.
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.”
Health Secretary Jeremy Hunt described the motion as “bogus”.
Sky sources understand the DUP will also support Labour on another non-binding motion calling for this year’s rise in university tuition fees to be scrapped.
It raises the likelihood of an embarrassing double defeat for the Government, highlighting the vulnerability of Mrs May’s minority administration.
The votes are outside a confidence and supply agreement the DUP signed with the Tories in the wake of June’s snap election, the result of which left the Conservatives looking for the Northern Ireland party’s support after losing their majority.
The Government has promised an extra £1bn for Northern Ireland over the next two years as part of the agreement.
Earlier on Wednesday, during Prime Minister’s Questions, Mrs May suggested some police officers had seen an increase in their take-home pay of 32% above inflation over the past seven years.
She pointed to “progression pay” for time served, annual raises and increases in the tax-free personal allowance since 2010/11.
Labour leader Jeremy Corbyn attacked the Government’s offer to police and prison officers as a “pay cut in real terms” due to the rate of inflation.
He also demanded the Prime Minister guarantee the pay rises announced this week would not be funded by cuts to services.