Sinn Fein will meet Theresa May face-to-face, to raise fears that her planned deal with the Democratic Unionist Party (DUP) will prevent the restoration of power sharing in Northern Ireland.

Gerry Adams, the party’s president, and Michelle O’Neill, its Stormont leader, will be in Downing Street on Thursday, even as the prolonged Conservative talks with the DUP grind on.

“I will be making it very clear that any deal between the Tories and the DUP cannot be allowed to undermine the Good Friday and subsequent agreements,” Ms O’Neill said in a statement.

Both London and Dublin must “recommit to the word, spirit and implementation of the Good Friday Agreement” if power sharing at Stormont is to be re-established, she said.

She added: “Progress will not come from a deal between the DUP and Tories to prop up a Government in Westminster with an austerity and Brexit agenda.”

Concerns have been raised, including by Sir John Major, that the Government can no longer be the necessary “honest broker” in talks in Northern Ireland, if it is formerly allied with the DUP.

The former Prime Minister warned of a return to violence, pointing to the danger posed by “hard men still there, lurking in the corners of communities”.

Downing Street insisted tomorrow’s talks will focus on restoring the Stormont Assembly. The Prime Minister will also meet the other Northern Ireland parties, the SDLP, the Ulster Unionists and the Alliance.

The announcement came after the DUP postponed any agreement to prop up the Tories at Westminster because of the Grenfell Tower fire tragedy.

No 10 has refused to confirm the further delay – and the Prime Minister refused to answer questions about the negotiations as she left Downing Street.

The 1998 Good Friday Agreement commits the UK and Irish Governments to demonstrate “rigorous impartiality” in all their dealings with the different groups in Northern Ireland.

Sinn Fein has declined to say whether it might challenge a Tory-DUP tie-up in the courts, but Alex Maskey, a Sinn Fein member of the Belfast Assembly, said it could “prove to be reckless”.

Power sharing at Stormont collapsed in March over the DUP’s refusal to hold a public inquiry into a renewable heating funding scandal.

In the talks to restore the assembly, which began on Monday, issues such as the introduction of an Irish language act, supported by Sinn Féin but opposed by the DUP, could also prove crucial.

Meanwhile, the DUP said diary commitments meant the conclusion of a “confidence and supply” agreement at Westminster for could be delayed until next week.

Under the deal, the party would offer support for key votes, such as backing the Queen's Speech and the Budget, and – crucially – in any votes of no confidence.

The Queen’s Speech is now expected to be delayed from next Monday until the following week and the start of the Brexit talks – also penciled in for next Monday – is also likely to be put back.

Even without the impact of the London fire, the talks were said to be making “slower progress”. Downing Street said only that they were “ongoing”.

The DUP is known to be pushing for a cut to air passenger duty and increased infrastructure spending, as part of any agreement.