Image copyright Reuters Image caption DUP MP Sir Jeffrey Donaldson and Tory Chief Whip Gavin Williamson signed the deal in June

Parliament will need to approve the provision of £1bn of extra funding for Northern Ireland, according to lawyers acting for the government.

The money was negotiated by the DUP in June as part of its confidence and supply deal with the Conservatives.

But lawyers said it has yet to be made available and any additional payments must be authorised by parliament through the normal budgetary process.

"No timetable has been established" for the release of the money, they added.

Image copyright AFP Image caption Gina Miller said she was 'stunned' to learn that the cash deal had to get parliamentary approval

The comments came in legal correspondence between the government, businesswoman Gina Miller and a trade union - the Independent Workers Union of Great Britain - who have been seeking to challenge the DUP deal payments in court.

They claimed that the £1bn funding was improper and discriminatory.

The government rejects the claimants' challenge.

Ms Miller told the BBC's Stephen Nolan Show on Monday she was "stunned" that the DUP cash required parliamentary approval and that this aspect of the deal had not been widely known.

Image caption The DUP agreed to support Theresa May's minority government as part of the deal

She said she assumed the DUP and the Conservatives could push the extra payment through.

But she questioned whether some Conservatives who are unhappy with the DUP deal, such as the Scottish Conservatives, might take "disruptive" action.

Ms Miller said she did not see any space in the government's legislative programme for a separate vote on the DUP cash and wondered whether the payment could be included in the budget in November.

DUP MP Sammy Wilson said that parliamentary approval was the "normal process" for money being allocated by the government and that there is "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."

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".