THE controversial £1 billion deal struck by Theresa May with the Democratic Unionist Party (DUP) to gain a House of Commons majority on key votes will have to be approved by Parliament.

The Treasury solicitor said in response to a legal letter from campaigner Gina Miller and a trade union that the investment package for Northern Ireland “will have appropriate Parliamentary authorisation” through the normal budgetary process.

Miller and the Independent Workers Union of Great Britain (IWGB)have been seeking to challenge the deal in court, claiming the funding was improper and discriminatory.

An earlier legal action by Miller forced the UK Government to give MPs and peers a vote on triggering Article 50, which formally began the process of Britain withdrawing from the EU. She said yesterday she was “stunned” that the DUP cash required parliamentary approval and that this aspect of the deal had not been widely known.

The Prime Minister, she said, should have made clear at the time that it would need Parliament’s approval.

“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 £1bn of taxpayers’ money for Northern Ireland could only be handed over following Parliamentary approval,” she said. “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,” she added.”

In the letter to Miller, the Government said that no timetable had been set for making the Northern Ireland payments.

The money was negotiated by the DUP in June as part of its “confidence and supply” deal with the Tories. Some Tory backbenchers have expressed their unease about the agreement with the socially conservative DUP which sees its 10 MPs back the Government on key votes.

Jason Moyer-Lee, general secretary of the IWGB, 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.”

DUP MP Sammy Wilson said parliamentary approval was the “normal process” for money being allocated by the Government and that there was “the promise” that the money will come.

“The Government can’t spend money without parliamentary approval and, since the money will be spent year-on-year, we would expect to see that in the estimates for Northern Ireland on a year-on-year basis,” he said.

The Government’s lawyers have indicated any additional payments are likely to be included in the “main or supplementary estimates of the Northern Ireland Office for the financial year in which they are made”.

Peter Dowd, Labour’s shadow chief secretary to the Treasury, said Chancellor Philip Hammond needs to reveal where the money for the deal is coming from.

“The Treasury’s admission of the need for a parliamentary vote on the £1bn DUP bribe is a clear indication that the Conservatives’ chaotic attempts to circumvent Parliament must come to an end,” he said.

“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 power sharing.”