The UK Government should legislate to remove the Northern Ireland Assembly's powers over welfare benefits, former first minister Lord Trimble has said.
The Northern Ireland Executive was warned last month that delays in implementing welfare reforms introduced in Westminster could cost the province £5 million a month by early next year.
Tens of millions of pounds may need to be found from elsewhere in the Executive budget if the block grant from London is cut to compensate for a failure to deliver savings on the cost of benefits, work and pensions minister Mike Penning said.
Lord Trimble warned failure to accept "parity" and stick to Westminster policies on welfare would open a "Pandora's box".
He said: "I do not have to remind people in Northern Ireland that parity was hard won and is of huge importance to poor and unemployed persons.
"The Stormont parliament stuck firmly step by step to UK national welfare policies, whether they liked them or not, and resisted opportunist suggestions from some of its own ranks to depart from parity.
"The Northern Ireland Executive has done that until now.
"I would say to those in Belfast who are seeking regional variation on this that they have to bear in mind the consequences that would flow from it.
"If the door is open to regional variation, it could be a two-way street and it could apply to other things as well - what comes to mind is things like public sector pensions and pay.
"So I would suggest to the Northern Ireland Assembly that they should close this Pandora's box as quickly as possible."
Lord Trimble, who was first minister in two spells between 1998 and 2003 as an Ulster Unionist, sits in the House of Lords as a Conservative peer.
In second reading debate on the Northern Ireland (Miscellaneous Provisions) Bill, he said welfare was not devolved in Scotland and Wales and although the "anomaly" could be tolerated if Northern Ireland stuck to parity, now it had departed from that the matter "should be addressed properly".
He said if there was legislation to bring the power back to Westminster, he doubted there would be "serious opposition at Stormont".
"I suspect the reason Sinn Fein has raised the issue of these welfare changes is because of its embarrassment at the contrast between its bitterly opposing austerity in Dublin while imposing it in Belfast," he said.
"I expect privately they would be relieved if this burden was removed from them."