FILE PHOTO - Campaigner Gina Miller arrives at Westminster Magistrates Court in London, Britain July 10, 2017. REUTERS/Neil Hall

LONDON (Reuters) - Britain’s government has been threatened with legal action by an anti-Brexit campaigner and a union over the deal Prime Minister Theresa May struck with a Northern Irish party to keep her Conservatives in power after her botched election last year.

Ministers have been sent a legal letter warning that 50 million pounds of funding for the province, part of a 1 billion pound deal agreed between May and the Democratic Unionist Party (DUP) - was unlawful because it was made without parliamentary agreement.

The challenge has been brought by Investment manager Gina Miller, who successfully won a court battle in 2017 to force the government to seek parliamentary approval before starting divorce talks with the European Union, and the Independent Workers’ Union of Great Britain (IWGB).

“It beggars belief that this government is once again putting itself above the law and seeking to undermine the normal constitutional and legal processes,” Miller said in a statement.

“Spending public money requires proper parliamentary scrutiny and accountability – and the making of these payments is no different.”

May called a snap election last June but lost her parliamentary majority and now relies on support from lawmakers in the small DUP to govern.

Miller and the IWGB say the government have broken an assurance given in August last year that payments to Northern Ireland could not be made without prior parliamentary approval.

Their lawyers have sent a letter demanding a response from ministers by Feb. 2 that any payment allocated to the British province will be returned and no further sums will be made without the agreement of lawmakers, or they will begin court action against the government.