Jeremy Corbyn is to promise that a Labour government would investigate reorganising the UK on federal lines as part of changes to reduce Westminster’s dominance, the Guardian can reveal.

In a major shift in the party’s thinking, its general election manifesto will say that a new constitutional convention it is establishing later this year will “consider the option of a more federalised country”.

The official manifesto, due to be published on Tuesday, will also commit Labour to the eventual goal of scrapping the House of Lords and replacing it with an elected senate, after first cutting its size, and to extending the powers of regional and local government across the UK.

The federalism proposal, absent from the draft version of the manifesto leaked last week, follows a swelling of support within Labour for devolving power after successive defeats in Westminster, Scottish and local elections, and the Brexit vote in last year’s EU referendum.

The case for federalism has been pushed hard by Scottish Labour’s leader, Kezia Dugdale, in an effort to weaken support for Scottish independence and to challenge the heavily unionist stance taken by the Conservatives, which has attracted some working class voters in Scotland.

In an effort to reinforce that message, the manifesto will confirm that Labour is to oppose the second independence referendum being sought by Scotland’s first minister, Nicola Sturgeon.

“Holding another referendum on leaving the UK is the wrong thing to do for Scotland’s economy, especially when there is so much economic uncertainty from the Tories’ plans for a reckless Brexit,” Corbyn said.

The final version of the manifesto has dropped a pledge to “check the privileges of the elite and give power back to the people”, but says the constitutional convention will invite proposals for “extending democracy”.

It adds: “This is about where power and sovereignty lies – in politics, the economy, the justice system and in our communities. The convention will look at extending democracy locally, regionally and nationally, considering the option of a more federalised country.”

The proposals, which echo long-held calls for a federal UK from the Liberal Democrats and a recent all-party constitutional reform group chaired by the Tory peer Lord Salisbury, were firmed up after a meeting of senior Labour figures in Cardiff in March.

The so-called devolution taskforce, chaired by the former Labour prime minister Gordon Brown, included Dugdale; the Welsh first minister, Carwyn Jones; Jon Trickett, a key Corbyn adviser on political reform in England; Andy Burnham, now the mayor of Greater Manchester; the mayor of London, Sadiq Khan; the former deputy prime minister John Prescott, and other local government leaders.

It is understood the manifesto does not include specific proposals for reforming the voting system or for cutting the number of MPs in the Commons, but proponents of federalism within the party believe the case for far-reaching constitutional reform has become more pressing following the vote to leave the EU.

The manifesto argues that decentralising power from London could help assuage resentment among former Labour voters about the unaccountability of Brussels and Westminster that fed the rise of Ukip. Critics of greater devolution within Labour point out that antipathy to regional assemblies has historically been greatest in Labour areas such as north-east England where Ukip’s influence surged before last year’s EU referendum.

Dugdale said she believed a constitutional convention would transform access to political and economic power after the UK left the EU. “I believe that moving political power out of Whitehall isn’t just a constitutional convenience, but an economic necessity.

“The threat to the future of the UK is very real. Our country is more fragile today than it has ever been, even in the days before the last independence referendum. The solution is not independence, which would lead to turbo-charged austerity in Scotland.”