THERESA May was condemned yesterday for signing a “shabby and reckless” £1 billion deal with the DUP to prop up her minority Conservative government which the two sides insisted would “strengthen the Union”.

Labour Party leader Jeremy Corbyn said the agreement was “not in the national interest”, while shadow foreign secretary Emily Thornberry said it would put the Northern Ireland peace process at risk.

“Austerity has failed,” said Corbyn. “Cuts to vital public services must be halted right across the UK, not just in Northern Ireland.

“The Government must immediately answer two questions – where is the money for the Tory-DUP deal coming from and will all parts of the UK receive the much-needed additional funding that Northern Ireland will get as part of the deal?”

He added: “This Tory-DUP deal is clearly not in the national interest but in May’s party’s interest to help her cling to power.”

Thornberry said: “This is a shabby and a reckless deal which has taken the Government at least £1bn to buy, and whose true cost for the future of peace in Northern Ireland could be infinitely higher.”

Thornberry challenged First Secretary of State Damian Green to publish legal advice about the deal’s impact on the Good Friday Agreement and pressed him to set out where the extra funding was coming from.

She added: “During the election he was fond of telling interviewers that there was no magic money tree. So what has happened today? Has he found the key to the secret garden?”

The Prime Minister announced she had reached an agreement with the DUP yesterday morning following 18 days of talks with Arlene Foster’s party.

The deal signed in No 10 Downing Street also saw the Conservative Party formally abandon plans to abolish the triple-lock protection for state pensions and means-test the Winter Fuel Payment during this Parliament.

Under a “supply and confidence” arrangement intended to last for the full Parliament, the DUP guarantees that its 10 MPs will vote with the Government on the Queen’s Speech, the Budget and legislation relating to Brexit and national security.

Together with the 317 Tory MPs who remain following May’s disastrous decision to call a snap election, this will give the Prime Minister just enough support to clear the 326 threshold required for an absolute majority in the House of Commons, ensuring her victory in key divisions.

The Welsh First Minister Carwyn Jones described the deal as “a straight bung to keep a weak Prime Minister and a faltering Government in office”.

He added: “Only last week we were told that the priority was to build a more united country, strengthening the social, economic and cultural bonds between England, Northern Ireland, Scotland and Wales.

“This deal flies in the face of that commitment and further weakens the UK and, as currently drafted, all but kills the idea of fair funding for the nations and regions.”

Former Treasury boss Sir Nicholas McPherson also warned that the DUP were likely to ask for more money.

“£1bn for Ulster is just a down-payment. DUP will be back for more – again and again. They have previous in such matters,” he tweeted. Speaking after talks in No 10, May said the two parties “share many values” and the agreement was “a very good one”.

Foster said she was “delighted” with a package which includes £1bn of new funding for infrastructure and health spending, along with enhanced flexibility on almost £500 million of previously allocated cash.

The agreement will remain in place for the length of this Parliament – due to end in 2022 – and can be reviewed “by the mutual consent of both parties”, the document says.

The DUP’s support in votes which are not covered by the confidence and supply arrangements will be agreed “on a case-by-case basis”.

May insisted the Government remains committed to the Northern Ireland peace process under the terms of the Good Friday Agreement.

Responding to the deal, Sinn Fein President Gerry Adams, whose party is in talks with restoring powersharing at Stormont after the administration collapsed earlier this year, said it was vital Stormont was re-established so the funds would be distributed fairly.

If negotiators miss Thursday’s deadline for a new agreement, they face the prospect of direct rule being reimposed from Westminster after months of talks failing to break the impasse.