Campaigner Gina Miller is threatening court action over payments to Northern Ireland under the £1 billion deal with the DUP to prop up the minority Conservative Government.
Lawyers acting for Ms Miller and the Independent Workers Union of Great Britain have issued a legal letter to the Government in relation to £50 million made available in this financial year to address immediate health and education pressures.
They argue that this breaches a previous assurance that cash support offered at the time of last year's post-election agreement would not be spent until it has been approved by Parliament in a vote.
And they warned that if ministers do not provide assurances by February 2 that they intend to recover the money, they will take the issue to court.
Investment manager Ms Miller previously defeated the Government in a Supreme Court case to win MPs the right to vote on the Article 50 letter triggering Brexit.
She said: "It beggars belief that this Government is once again putting itself above the law and seeking to undermine the normal constitutional and legal processes.
"Spending public money requires proper parliamentary scrutiny and accountability - and the making of these payments is no different.
"The Government excuse for making these payments without parliamentary approval is that the monies were to address immediate health and education pressures in Northern Ireland.
"What about the immediate and pressing health and education needs in the rest of the United Kingdom? There is a demand for more social spending in all parts of the UK. This payment is more about keeping this Government in power."