The Democratic Unionist Party have agreed in principle a “confidence and supply” deal to support a Conservative government, it has been announced.
Theresa May was left eight seats short of an overall majority in the general election, while the DUP won 10 seats.
Tory chief whip Gavin Williamson went to Belfast on Saturday for talks with the Northern Irish party.
Downing Street said the details of the outline deal would be discussed at a cabinet meeting on Monday.
Any agreement would come into force when Parliament returns next week.
A “confidence and supply” deal is not a full coalition, but an agreement which sees the smaller party support the larger one in key votes such as the budget.
A No 10 spokesman said: “We welcome this commitment, which can provide the stability and certainty the whole country requires as we embark on Brexit and beyond.”
Liberal Democrat leader Tim Farron called on Mrs May to make the details of the deal public “immediately”.
He said: “The actions of this government will have profound implications for the Brexit negotiations and the future of our country.
“At such a critical time, the prime minister must be clear with the people about the deal she has stitched up with the DUP behind closed doors.”
Sinn Féin’s northern leader Michelle O’Neill said the DUP had “betrayed the interests of the people” of Northern Ireland.
She said: “They have achieved little propping up Tory governments in the past and put their own interests before those of the people.
“This new arrangement between the DUP and the Tories will be transitory and will end in tears.”
The DUP are pro-union in UK terms, pro-Brexit and socially conservative.
The party’s 2017 manifesto set out their position on Brexit and other issues, including some where there is common ground with the Tories and others where there is not.
The party’s calls for increases in the personal tax allowance, continued rises in the national living wage, renewal of the Trident nuclear deterrent and revisiting terrorism laws are all similar to Conservative policy.
But they differ in areas including abolishing air passenger duty, cutting VAT for tourism businesses and maintaining a triple lock on pensions.
‘Full range of possibilities’
Newsnight political editor Nick Watt said the DUP and Conservatives had been “looking at the full range of possibilities” during Mr Williamson’s visit.
The parties discussed everything from just an informal undertaking all the way through to a full coalition agreement.
He added: “I am told that what the DUP are looking at is securing benefits. So keep the pension triple lock and preserve universal benefits such as the winter fuel allowance for pensioners.
“What that would allow the DUP is to say: we are not just acting in the interests of Northern Ireland. We are true unionists. We are acting in the interest of the whole of the United Kingdom.”
Analysis by political correspondent Gary O’Donoghue
The clock is ticking for Theresa May. She needs to conclude a deal with the DUP in the next week or so ahead of the Queen’s Speech, which will set out the new government’s agenda.
That takes place on Monday 19 June – the same day Brexit negotiations are due to start.
The DUP and its 10 MPs are in a very strong position. It’s all their Christmases rolled into one and they will make sure they leverage as much as they can from their advantage.
Money for Northern Ireland will undoubtedly be part of their demands, and Mrs May will expect that. But trickier will be any demands they have about the implementation of Brexit in Northern Ireland – in particular the DUP’s determination to maintain a soft border with the south.
Another potential problem is the planned restart of negotiations for power-sharing in the province.
Typically the British government tries to act as an honest broker between republicans and unionists. But if Mrs May is doing a deal with the DUP, that could make it harder to reach an agreement with Sinn Fein.
Alastair Ross, a former DUP member of the legislative assembly in Northern Ireland, told BBC Radio Ulster’s Inside Politics he would not expect a deal to last more than 12 or 18 months.
Speaking before the supply and demand announcement he said: “They (the DUP) would perhaps support them (the Conservatives) in a budget and the Queen’s Speech, but allow themselves the flexibility to take different positions to the Conservative Party if it’s in the interests of Northern Ireland to do so.
Former Ulster Unionist Party leader Lord Empey warned Theresa May that she should get feedback from her MPs before entering into an arrangement with the DUP.
He told the programme: “My advice to the prime minister would be to wait until her backbenchers get into Parliament next week and she tests opinion because, I can tell you I’m there every week, these people will not want to be held to ransom by any regional party whichever one it happens to be.”
What is a ‘confidence and supply’ deal?
This deal would see the DUP promise to back the government in votes of no confidence and supply – or budget – issues.
In return, the government would support or fund some of the DUP’s policies.
These deals tend to be loose and a long way short of a formal coalition.
Minority governments like this are not uncommon.
John Major survived without a majority in the dying days of his Tory administration in the mid-1990s.
Labour’s Harold Wilson and James Callaghan governed with minorities for much of the 1970s.
But these governments can be quite constrained in what they can do, passing as little legislation as possible to avoid defeat.
They can also be unstable and short lived, if the deal between the parties breaks down and fresh elections have to be called.