"Austerity has failed," the Labour leader said. "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?
"This Tory-DUP deal is clearly not in the national interest but in May's party's interest to help her cling to power."
The Prime Minister claimed the agreement would be a "very, very good one," despite concern tying her government to the socially conservative DUP will drag the UK further to the political right.
Early indications showed Ms May was forced to ditch election manifesto commitments, promise £1bn of extra spending and even give lucrative new tax powers to Northern Ireland in order to secure the DUP's support.
The DUP leader, Arlene Foster, told reporters both parties had agreed there would be no change to the pensions triple lock and the universal nature of the winter fuel payment - both things Ms May had pledged to change at the election.
Details later published by the Government showed Downing Street has promised £200m extra per year for two years to pay for new infrastructure and a further £75m per year for boosting broadband.
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