The DUP said it would back the government in any confidence votes and to pass budgets
British Prime Minister Theresa May's Conservatives struck a deal with an ultra-conservative party on Monday that will allow them to govern in exchange for an extra £1 billion in funds for Northern Ireland.
The controversial pact with the Northern Irish Democratic Unionist Party comes after May ended up losing her parliamentary majority in an election earlier this month that she had called to boost her support for Brexit talks.
Under the terms of the agreement, Northern Ireland will receive an extra £1.0 billion (1.1 billion euros, $1.3 billion) from the state over two years in exchange for DUP supporting May's Conservatives.
The DUP said it would back the government in any confidence votes and to pass budgets, as well as supporting it on Brexit-related legislation.
For any other parliamentary votes, the DUP -- which has 10 MPs -- said its support would be a on a case-by-case basis.
The agreement was signed by senior Conservative and DUP officials in May's Downing Street office as May and DUP leader Arlene Foster looked on.
Opposition parties swiftly criticised it, with the Liberal Democrats leader Tim Farron calling it a "shoddy little deal".
"The nasty party is back, propped up by the DUP," he said in a statement.
Welsh First Minister Carwyn Jones of the main opposition Labour Party called the dea "outrageous" and "unacceptable" and said the money was a "bung" to keep a weakened prime minister in office.
The Independent news website said May was paying a "high price to cling on to power".
The Conservatives have 317 seats in the 650-seat parliament after the June 8 election and need the support of the DUP's 10 MPs to be able to govern.
The deal with the DUP will also prove controversial because of the party's opposition to gay marriage and abortion and concern that it could upset the fragile balance of the peace process in Northern Ireland.
"I welcome this agreement which will enable us to work together in the interests of the whole United Kingdom," May said in a statement.
Foster said: "This agreement will operate to deliver a stable government in the United Kingdom's national interest at this vital time."
The DUP supported Brexit but has emphasised the need to keep the border with the Irish republic open, and Foster said the deal would back a Brexit process "that supports all parts of the United Kingdom".
Foster said the extra money would be spent on infrastructure, health and education, benefitting the whole of Northern Ireland after concerns voiced by the republican Sinn Fein party, the DUP's rivals.
The deal will face its first test in parliament with a confidence vote expected on Thursday.
The main opposition Labour Party has said it will push for another general election.
Discussions between the Conservatives and the DUP began immediately after the election, stirring up further resentment against the embattled May who was left weakened by the embarrassing political setback.
Some DUP representatives have been criticised in the past for homophobic comments, climate change denial statements and sectarian rhetoric.
Ireland's former premier Enda Kenny has warned that a deal with the Protestant and pro-British DUP could upset Northern Ireland's fragile peace.
London's neutrality is key to the delicate balance of power in Northern Ireland, which was once plagued by violence over Britain's control of the province.
Northern Ireland's political parties are still locked in negotiations to form a semi-autonomous power-sharing executive for the province, nearly four months after local elections there.
If the parties cannot come to an agreement by Thursday, Northern Ireland may be returned to direct rule from London.