The Democratic Unionist Party’s £1 billion deal to prop up the Conservative government may end up costing the country far more because the DUP will be “back for more”, it emerged last night.

The Tories finally sealed a historic deal with the Northern Irish party which guarantees its 10 MPs will vote with the Government on key legislation, in return for which cash will go to Belfast for infrastructure, broadband, schools and hospitals.

But the £1billion payment - the equivalent of £33 for every taxpayer in the UK - could be only the start after DUP sources hinted that they will ask for more cash when the deal is “reviewed” in two years’ time.

The SNP reacted by immediately demanding more money for Scotland, while the Welsh government claimed it should be given £1.7 bn to achieve parity with Ulster. Sinn Fein, meanwhile, claimed the deal threatened the Good Friday Agreement.

After 18 days of negotiations, the DUP leader Arlene Foster flew to London yesterday to oversee the signing of the deal.

Theresa May chose not to sign the document, instead leaving it to the Conservative chief whip Gavin Williamson to do so at the Cabinet table in Downing Street.