Gordon Brown has issued a stark warning that the UK remains on “life support” because the Tory government has given up on saving the 308-year-old union as it shores up support by playing the English card.

In a powerful critique of David Cameron, as No 10 brings forward plans to limit the voting rights of Scottish MPs at Westminster, the former prime minister accuses the Tories of adopting an “insidious” approach to divide England and Scotland.

Writing in the Guardian, Brown says: “If the United Kingdom collapses, it will not be because a majority of Scots are hell-bent on leaving but because the UK government is giving up on saving it.

“No union can survive without unionists and, after an election in which, to head off Ukip, the Conservative and Unionist party presented itself as the English Nationalist party, it is clear that the union is on life support … It is London’s equivocation over Scotland that is becoming the greater risk to the UK.”

The intervention by Brown is one of his strongest attacks on the Tories over Scotland since the prime minister announced, within hours of the results of the independence referendum last September, that he would seek to restrict the voting rights of Scottish MPs. The former prime minister was so enraged by his successor’s early morning announcement that he telephoned the cabinet secretary, Sir Jeremy Heywood, to warn that Cameron was handing a gift to the SNP.

In his Guardian article, Brown accuses the Tories and the SNP of resorting to a “sectarian war of words” as they both push for Scottish and English vetoes. He cites the SNP’s so-called “quad lock”, which would mean that Britain could only leave the EU if the referendum is passed by a majority of voters in the UK and is also approved by a majority of voters in each of the four constituent parts of the UK.

Brown also points to the Tory plan to block Scottish MPs from voting on English-only matters at Westminster – the so-called English votes for English laws. The Times reported on Thursday that the prime minister, who may table the changes next week, has toughened up his plan to prevent Scottish MPs from English legislation even if it has a knock-on effect on the Scottish block grant.

Brown writes: “In the last few weeks SNP and Conservative politicians have descended into a sectarian war of words, raising the spectre of Scottish and English ‘vetoes’ and suggesting there are irreconcilable differences between the two nations.”

The former prime minister argues that instead of challenging the SNP’s “dubious constitutional principle” about handing individual parts of the UK a veto in the EU referendum, the Tories are also playing a nationalist card in England by conjuring up a “Scottish menace”. He cites the Tory election poster which featured Nicola Sturgeon as Ed Miliband’s puppet master and the plan for the UK government to scrutinise the work of the Scottish parliament.

Brown writes: “Sadly, this tactic – to divide and rule and put party before country rather than to unite – is one that the Conservatives can return to again and again.

“It reveals a bigger truth: that while Scotland has not yet written off Britain, the Conservatives are starting to write off Scotland.”

The former prime minister calls for the union to be saved through the creation of a cross-party constitutional convention to bind the UK’s four constituent parts with a common statement of values. He also wants an agreement to bind all future UK governments to honouring “basic social and economic rights” such as healthcare and pensions.

Brown, who will further outline his thoughts on prospects for the union at the Borders book festival in Melrose on Saturday, concludes: “In a Britain where the battered forces of progressive opinion urgently need to regroup and find common purpose, a constitutional guarantee to the citizens of all four nations could be the best way, and perhaps the last chance, to show that there is a clear and explicit vision of how the peoples of Scotland, England, Wales and Northern Ireland can achieve more together through cooperation and sharing than we can ever do by breaking apart.”

Downing Street defended the prime minister’s plan to limit the voting rights of Scottish MPs, which will be achieved through a simple change to parliament’s standing orders through a vote in the commons. Salmond, the former Scottish first minister, says it is wrong to make such a fundamental constitutional change without legislation.

Cameron’s spokeswoman said: “The government will bring forward its proposals in this parliamentary session. What we are seeking to do with these proposals is to balance the principle of English consent for English measures with MPs from all parts of the country continuing to deliberate and vote together. That is what we will seek to achieve with these proposals when we set them out in this parliamentary session.”