Conservative leader David Cameron's "English votes on English laws" policy could lose him potentially vital DUP support in any future hung parliament at Westminster.
This became clear yesterday after a week in which the post-devolution "English question" sparked heavyweight exchanges between Mr Cameron and prime minister Gordon Brown in the House of Commons.
DUP MP Sammy Wilson has intervened twice this week in the ongoing Commons debate on the Queen's Speech, siding with Justice Secretary Jack Straw against one Tory proposal for a grand committee of English MPs to determine English-only legislation. And the East Antrim MP last night told The Irish Times he believed the issue could be a deal-breaker should Mr Cameron at some future point need DUP support either to form or sustain a Conservative government.
Many commentators believe a hung parliament could be the outcome of the next British general election.
Mr Wilson also confirmed that the Tories have been discreetly wooing the DUP at Westminster, and would regard them as natural allies should the opportunity present itself to replace the Labour government.
However, he said: "My own view is that we would find it very difficult to support them [the Conservatives] if they were going to introduce a measure that would be very detrimental to the Union."
The Ulster Unionists, who currently hold only one Westminster seat, are similarly opposed to any move that would create two categories or classes of MP.
However the Conservatives insist that that situation has already arisen, with Scottish, Welsh and Northern Ireland MPs able to vote on a range of issues affecting England while unable to influence the same issues in respect of their own devolved constituents.
Mr Wilson said the Conservatives were worried about the possibility of losing some Westminster votes, while observing: "Some people might think it emotive to say so but people in Northern Ireland actually died because they believed in the Union with Great Britain".
Dismissing the grand committee option as "very short-sighted" and "opportunistic", Mr Wilson said he believed the Conservatives were now being "reckless with the Union", having originally seized upon the issue only "as a stick with which to beat Gordon Brown".
In the Commons on Tuesday night former Conservative minister John Redwood agreed with MP Angus Robertson that Scottish MPs should follow the example of the Scottish National Party members and "abstain on matters that are solely English".
However, Mr Wilson told Mr Redwood: "As a staunch unionist, I find that argument rather disturbing.
"Although devolution did not apply to Northern Ireland for many years, part of the contract of the Union was that decisions were made in this house by people who often represented parties that did not even organise in Northern Ireland.
"In the absence of devolution in England, does that contract not also apply to people in England in the sense that all honourable members should make decisions about what happens in this part of the United Kingdom?"