The Democratic Unionist Party will oppose the Blair government's proposal to introduce ID cards for every citizen of the United Kingdom, if the British database is to be shared with the Irish authorities.
This emerged last night as DUP sources confirmed the party is in negotiation with ministers about its possible support for the controversial Bill, which is due to receive its second reading in the House of Commons tomorrow.
The assumption in London is that if the British scheme gets parliamentary approval, the Irish Government will be obliged to follow suit if the Common Travel Area between the UK and the Republic is to be maintained.
Minister for Justice Michael McDowell has declared himself "very reluctant" to introduce compulsory ID cards, arguing they would fundamentally change the nature of the relationship between the citizen and the police.
British prime minister Tony Blair, on the other hand, has dismissed civil liberties concerns, while selling the proposed ID card as a necessary protection against terrorism, benefit fraud, illegal immigration and identity theft.
British opponents of the Bill received a boost yesterday when the largest trade union, Unison, - representing officials who would be charged with implementing the scheme - came out against it.
The Conservatives and Liberal Democrats are hoping Labour rebels will help kill off the Bill during its later parliamentary stages.
There are also suggestions that British home secretary Charles Clarke could drop the proposal, if it can be shown that it will be too costly or that the technology would not work.
A London School of Economics report to be published today is expected to suggest the final cost of the scheme could be £18 billion, some three times the British government's estimate.
However, Mr McDowell discussed the British plan with Mr Clarke on the margins of an EU summit earlier this year.
And a spokeswoman for the Department of Justice confirmed that discussions with London are ongoing about a range of practical issues, including cost and the development and possible sharing of the technology.
The spokeswoman did not think these still "tentative" discussions had extended to the question of a shared database.
However, it is understood Mr McDowell has specifically raised with Mr Clarke the position of people crossing the Border on an hourly basis, and that of some 700,000 people in Britain who hold Irish passports.
The London/Dublin discussions have also touched on the politically sensitive question of whether British ID cards would carry the Union Jack.
Official sources say there would have to be "cross recognition" of any British and Irish ID cards in respect of people travelling between Northern Ireland and the Republic.
However, DUP deputy leader Peter Robinson told The Irish Times his party's nine MPs would oppose the plan, if the volume of traffic between Britain and Ireland necessitated giving "a foreign country" access to the database holding confidential details on every British citizen.
The proposal to introduce ID cards promises the first serious challenge to Mr Blair's reduced majority.
It is thus the first chance for the DUP - now the fourth party in the House of Commons - to test its post-Westminster election leverage.
Its expectations for appointments to the House of Lords, and the demand that Westminster's Northern Ireland Grand Committee hold sittings in Belfast, are thought likely to feature in the DUP discussions with ministers about Mr Clarke's Bill.