Wales loses under Tory 'power bribe' for DUP, warns Jones
Conservative UK ministers will find an "underhand way" to give Northern Ireland more money at Wales' expense, First Minister Carwyn Jones has said.
It follows reports the Democratic Unionist Party wants £2bn extra funding from a Tory-DUP deal at Westminster.
Mr Jones described such a deal, to give Theresa May a majority in key Commons votes, as the "great power bribe".
Responding, Welsh Secretary Alun Cairns said UK ministers had a "pretty strong record" on funding for Wales.
After the Conservatives' failure to win a majority in the general election, they have been locked in talks with the DUP for support to help them survive as a minority government.
Sources told the BBC on Wednesday the DUP wanted £1bn investment in the Northern Irish health service and a similar figure for infrastructure projects, although a DUP MP called those figures "wide of the mark".
Mr Jones said: "Ah, the great power bribe.
"What we're seeing there is an attempt to provide more money for Northern Ireland and, if I were the DUP, I'd do the same thing.
"But the reality is you can't give more money to Northern Ireland and not trigger the Barnett formula, which would mean more money for Wales and Scotland."
The Barnett formula is a long-running mathematical calculation that distributes money to Wales, Scotland and Northern Ireland.
Mr Jones added: "I think every underhand way will be found to give more money to Northern Ireland at the expense of Scotland, Wales and indeed the English regions."
Welsh Conservative leader Andrew RT Davies has said "any potential incentives considered for one nation... must also be considered for Wales".
But Prof Gerry Holtham, an economist and former adviser to the Welsh Government, said the Barnett formula "works in one direction only" - to ensure spending changes in England are reflected in Wales, Scotland and Northern Ireland - and was "just a convention".
"If the government decides it's going to do a formula bypass, which it's done on many past occasions, that's what it does," he said.
"They will do what they can get away with, basically. If the pressure for fairness is politically significant then they might make some concessions but they really don't need to."
The DUP is also reported to be calling on the Treasury to scrap Air Passenger Duty (APD) in Northern Ireland.
Such powers have already been transferred to Scotland.
Mr Jones, whose Welsh Government owns Cardiff Airport, said: "It's completely irrational that Scotland and Northern Ireland would control Air Passenger Duty, as would England, but Wales wouldn't."
Mr Cairns said the current funding settlement for Northern Ireland was "very different" from that for Wales.
"Wales has £120 for every £100 that's spent in England, Wales has a funding floor that Northern Ireland doesn't have," he said.
"And Wales also has City Deals for the Cardiff Capital Region and the Swansea Bay City Region as well as a commitment to the North Wales Growth Deal - there isn't a City Deal in Northern Ireland.
"So that demonstrates we've got a pretty strong record and I hope that is recognised."
Pontypridd MP and Labour's Northern Ireland spokesman Owen Smith said any Tory-DUP deal had to be scrutinised.
But Westminster politics "can't inhibit getting the peace process up and running" in Northern Ireland, he said.
Speaking on BBC Wales' The Wales Report, he said: "Talking about parity between the sorts of treatment that Northern Ireland gets and other parts of the UK isn't what we've done in the past.
"Northern Ireland is a special case and it will always need special consideration."