�We�ve got to be understanding with the Scots�

Justice secretary Jack Straw will today offer the government's first major hint that Labour's Westminster policy on retaining the current constitutional status quo between Scotland and England is about to change.

Following recent comments by the Scottish secretary, Des Browne, that he was "relaxed" about Holyrood gaining powers from Westminster, Straw says on GMTV's Sunday Programme that England has "got to be careful" about how it uses its power within the union.

Straw, who holds responsibilities for constitutional affairs in Gordon Brown's Cabinet, warned that both the government and the English had to be careful "if it wants this Union to continue".

Describing England as the dominant part of the union, with 80% of the population which earned 85% of the money, he said Britain was stronger and the English better represented by being part of the union. "If it broke we would all be diminished," he said.

Gordon Brown has consistently said that the current post-devolution relationship between Westminster and Holyrood was working well and there were no plans to change the role and influence of Scottish MPs in Westminster. Nor were there plans to alter the devolution settlement of 1998.

But Straw's comments today will spark a new debate that Labour's commitment to the status quo could be about to change. Far from expressing content that the current system is working well, the justice secretary says: "We've got to be understanding with the Scots and Welsh, not indulgent."

The prospect of the Holyrood parliament being given more powers by Westminster under Brown's premiership seemed remote only months ago. But Straw's assurances that "power devolved is not power ultimately given away" suggests a softening of the stance that the current devolution settlement is set in political stone.

With the new leader of Labour in Scotland, Wendy Alexander, also stating that she favoured changes to devolution that would mean greater autonomy, Westminster appears to have got the message that managed change is better than a long slide towards independence.

Although Straw and Browne have both so far kept quiet about what precise constitutional changes would be acceptable, a general election campaign would bring a full constitutional debate out into the open.

Straw, who is also Gordon Brown's leadership organiser, said little to GMTV that will quash rumours that the prime minister of barely 100 days is about to announce a snap election that could take place at the end of next month.

If an election is imminent, Straw effectively kicked off the constitutional fight between Labour and David Cameron's Conservatives, saying that the Tory policy of "English votes for English laws" in the Commons would "break up the union".

Pointing to the 19th century debates on Irish Home Rule, where a similar approach was tried and led to the Irish revolt and eventual separation, Straw said: "You can see the same thing happening in Scotland if ever you got a Conservative Party seeking the same solution."

Straw said that if the Conservatives went down the road of English votes for English laws, they would "end up by breaking the Union, first by forcing an English parliament on the union, and then since no federation in the world has ever been able to operate where one unit is 85% of the total, breaking the Union altogether and that is in no-one's interests."