The controversial £1 billion deal the Tories struck with the DUP in order to gain a Commons majority on key votes will need to be approved by Parliament, it has emerged.
Responding to a legal letter from campaigner Gina Miller and the Independent Workers Union of Great Britain (IWGB), the Treasury solicitor said the investment package for Northern Ireland "will have appropriate Parliamentary authorisation".
Ms Miller, whose legal action forced the Government to give MPs and peers a vote on triggering Article 50 which formally began the EU withdrawal process, said Prime Minister Theresa May should have made it clear at the time of the deal that it would need the approval of Parliament.
She said: "It beggars belief that, neither at the time the Government sealed its dubious deal with the DUP in exchange for their votes in the Commons, nor at any point since, has the Government made it clear that the £1 billion of taxpayers' money for Northern Ireland could only be handed over following Parliamentary approval.
"We all need to know when the Government intended to come clean to Parliament, its parliamentary party, and the public.
"When was parliamentary time going to be found to authorise this payment? And did the DUP know the cheque the Government promised to pay might bounce?
"On the day the Government is asking MPs to grant it sweeping new powers, and in the week it is trying to pack parliamentary scrutiny committees to blatantly change the rules in their favour, MPs are entitled to wonder what else the Government may have 'forgotten' to tell them."
The Government said in the letter to Ms Miller that no timetable had been set for making the Northern Ireland payments.
Some Tory backbenchers have expressed unease about the "confidence and supply" agreement with the socially conservative DUP which sees its 10 MPs back the Government on key votes.
General secretary of the IWGB Jason Moyer-Lee said: "Many IWGB members' jobs depend on public money, like foster care workers and low-paid outsourced university staff.
"They are routinely told that there's no money available to improve their pay, holidays, and other terms and conditions they demand.
"Yet when it comes to keeping themselves in power, this Government's fiscal discipline quickly dissipates.
"There's undoubtedly a need for increased social spending throughout the UK but this should be on a basis of fairness; not self-serving party politics.
"As a result of our threatened legal action the Government has admitted that the money can only be approved by Parliament.
"It is now for Parliament and MPs to vote according to the interests of working people across the whole UK."
Labour's shadow chief secretary to the Treasury, Peter Dowd, said Chancellor Philip Hammond needs to reveal where the money for the deal is coming from.
He said: "The Treasury's admission of the need for a parliamentary vote on the £1 billion DUP bribe is a clear indication that the Conservatives' chaotic attempts to circumvent Parliament must come to an end.
"We do not begrudge Northern Ireland getting the spending that it needs, but people all across the UK are crying out for the investment, jobs, and decent pay which will only be delivered by the next Labour government."
A Downing Street spokesman said: "All UK Government spending requires parliamentary authorisation - generally via the Estimates and Supplementary Estimates process. Our focus in Northern Ireland is on restoring powersharing."