David Cameron's push for 'English votes for English laws' means Gordon Brown could be the last Prime Minister of the United Kingdom from a Scottish constituency, a Conservative MP has admitted, amid warnings the Union is "smashed".

Rory Stewart, the Tory MP and chairman of the Commons Defence Committee, called for care to ensure Scots were not barred from the top job.

Meanwhile, Labour 'guru' Maurice Glasman claimed that Mr Brown was already the last ever Scottish Prime Minister - and blamed the former Labour leader himself, saying he had "smashed' the Union.

Lord Glasman said that Mr Brown had broken the UK with his call for Scottish 'Home Rule' in the final days of his fight against independence.

Mr Cameron's surprise "English votes" announcement came less than an hour after confirmation of the independence result.

Earlier this week Labour attacked the move, saying it had made it more difficult to agree a deal on pledged extra powers for Scotland.

Asked at a fringe event at the Conservative party conference if "English votes" could make it impossible for a Scottish MP to become Prime Minister, Mr Stewart said: "This is a really big challenge. There is of course the theoretical possibility. That if one had a situation in which you consistently ended up with a majority that depended on Scottish MPs it would be difficult to have a Scottish Prime Minister, and I think that would be unfortunate." Mr Stewart said that he liked "intuitively" the conclusions of the McKay commission, which advocated indicative but not binding votes among English MPs.

Labour have accused Mr Cameron of trying to set up a separate English Parliament within Westminster, that he can control if even he loses the keys to Downing Street.

But Labour peer Lord Glasman said Gordon Brown had already made it impossible for a Scottish MP to be Prime Minister.

"What we have done is smashed the Union," he told the same fringe, organised by the Institute for Public Policy Research think tank.

"Gordon Brown stood up and made a speech after which it is impossible for any Scot to ever be Prime Minister again. And that is a tragedy for our country.

"If there is a union, there is a union. But you cannot have Home Rule and then equally be part of a union. So I think we need to step back here."

Yesterday Labour sources warned that Mr Cameron's move to restrict the voting rights of Scottish MPs had made it harder to reach a compromise on what level of income tax should be devolved to Holyrood.

The Conservatives are pushing for 100% of the levy to be given over to MSPs. But Labour warn that could make it difficult for any future Labour chancellor to get a Budget through.

Former Labour Trade Minister Lord Digby Jones said it would be a "farce" if Scottish MPs were allowed to vote on taxation affecting voters in Birmingham when English MPs could no longer impact on taxation in Scotland.