London: Ed Miliband has rejected David Cameron’s proposal to link further devolution to the Scottish parliament to a ban on MPs from north of the border voting on English-only matters at Westminster as a “back of the envelope, fag packet” calculation.

As Alistair Darling warned that the prime minister was in danger of breaking his solemn vow to the Scottish people, the Labour leader suggested he was open to the more modest idea of giving English MPs greater scrutiny of legislation south of the border.

Miliband also indicated that the prime minister gave him no indication in their discussions during the Scottish referendum that he would table plans to limit the voting rights of Scottish MPs. Their discussions led to a “vow” in Tuesday’s Daily Record in which the leaders of the UK’s three main political parties agreed to “deliver change for Scotland”.

The row over the constitutional future of Scottish MPs threatened to overshadow the opening of the Labour conference in Manchester, which is meant to focus on the party’s plans to help low-paid workers.

Alex Salmond, the outgoing Scottish first minister, accused the prime minister of tricking the voters of Scotland by reneging on his commitment delivered during the referendum campaign amid fears that the pro-UK side would lose. Darling warned that anyone who “welches” on the deal would pay a “very heavy price for years to come”.

But the prime minister, who said on Friday morning that further powers to the Scottish parliament must be introduced in tandem with blocking Scottish MPs from voting on English-only matters, intensified the pressure on Labour to support his plan. Cameron, who will discuss his plans to agree the changes by the time of next year’s general election with rightwing critics at Chequers on Monday, warned that if Miliband blocked his proposal he would have to “explain to the people of the rest of the UK why they shouldn’t have the same powers as we are rightly devolving to the people of Scotland”.

Miliband insisted that such major constitutional changes should be introduced with care as he reiterated his call for a constitutional convention to consider the matter next autumn. He suggested he would limit the changes to giving English MPs greater scrutiny of legislation relating solely to England. Labour would struggle to govern without the support of its 41 Scottish MPs.

The Labour leader told the Andrew Marr Show on BBC1: “I am open to the idea of greater scrutiny of legislation by English MPs. But we can’t do it in a back of the envelope, fag packet way. We have spent two years trying to keep our country together. Let’s have a proper constitutional convention. Let’s look at these issues. But let’s not drive our country apart because David Cameron thinks it is an opportunity for him to do it.”

Miliband insisted that the West Lothian question, which asks how Scottish MPs can be allowed to vote on English-only matters when English MPs have no say over devolved matters north of the border, needs careful consideration. He said: “There isn’t a simple answer to this question. We have been wrestling with this issue for 120 years ever since we were talking about Irish Home Rule around a century ago. When you devolve power to some parts of the country in different ways to others it throws up these questions. So London MPs vote on transport issues outside London but English MPs outside London don’t vote on transport issues in London because they are devolved.”

There was further pressure on the prime minister as Darling insisted that there could be no link between further powers for Holyrood and limiting the voting rights of Scottish MPs at Westminster. The leader of the Better Together campaign told the Andrew Marr Show: “The agreement reached by the three parties, as far as I am concerned, is non-negotiable. It was promised, it has got to be delivered. Anyone who welches on that will pay a very heavy price for years to come. It is simply non-negotiable.

“There is a separate issue about what further constitutional change comes to the UK. But, just be very clear about this, you cannot hold up or delay in any way at all what was promised. The three leaders gave an absolute commitment and I am confident they will deliver on it.”

Salmond said that ‘No’ voters in the referendum would feel they had been “misled”, “gulled” and “tricked”. The first minister told the Sunday Politics on BBC1: “I am actually not surprised they are cavilling and reneging on commitments, I am only surprised by the speed at which they are doing it. They seem to be totally shameless in these matters. The prime minister wants to link change in Scotland to change in England. He wants to do that because he has difficulty in carrying his backbenchers on this and they are under pressure from Ukip.

“The Labour leadership of course are frightened of any changes in England which leave them without a majority in the House of Commons on English matters. I think the vow was something cooked up in desperation for the last few days of the campaign and I think everyone in Scotland now realises that.”

But the cabinet pressure on the prime minister was highlighted when Chris Grayling, the justice secretary, warned of a backlash if English-only votes were not implemented. In an article for the Sunday Telegraph he wrote: “That would be a travesty of democracy, and would be regarded with fury by the English. In Scotland, justice is wholly devolved. As secretary of state and lord chancellor, I have virtually no role there. But Scottish MPs today can still vote on my proposals to toughen the justice system in England and Wales. That clearly cannot continue.”

— Guardian News & Media Ltd