Gordon Brown promised the "most comprehensive programme of constitutional reform for a century" as he sought to seize the initiative in the General Election campaign today.
The Prime Minister said the Labour Party's election manifesto would "chart a course" towards a written constitution - with a commitment to fixed-term parliaments - as part of a wide-ranging plan to restore trust in politics.
Earlier Mr Brown and David Cameron clashed again over National Insurance at the final Prime Minister's Questions of the current parliament.
The Conservative leader claimed the Government's planned increase would "wreck" the recovery.
And Labour's clash with business leaders who opposed the rise threatened to escalate as Mr Brown repeated Business Secretary Lord Mandelson's claim that they had been "deceived" by the Conservatives.
In his first set-piece speech of the campaign, Mr Brown said that fundamental reform of the political system was essential to rebuilding public trust in the wake of the expenses scandal.
As well as pledging fixed-term parliaments - ending the historic power of the prime minister to choose the timing of future elections - he promised referendums on voting reform for Westminster elections and final reform of the House of Lords.
Other measures would include a ban on MPs working for lobbying companies, giving voters the power to get rid of MPs who are guilty of gross financial misconduct, a right of petition to trigger Commons debates on issues of public concern and a free vote for MPs on lowering the voting age to 16.
"All politicians, of every party and at every level, must acknowledge that there has been a fundamental rupture in the bond of trust between those who serve, and those who they are sworn to serve," he said.
"So I am asking the British people for a mandate to undertake the most comprehensive programme of constitutional reform in this country for a century."
He added: "I would take no pride in walking through the door of No 10 again, take no joy in victory, if it comes without a mandate to get rid of the old discredited system of politics."
Mr Brown's promise of a referendum on voting reform for Westminster elections - to be held by October 2011 - has been widely seen as an overture towards the Liberal Democrats in the event of a hung parliament.
Labour's proposed alternative vote system does not, however, go as far as the full system of proportional representation favoured by the Lib Dems.
Lib Dem leader Nick Clegg reacted coolly to the proposal, insisting neither Labour nor the Conservatives could be trusted when it came to constitutional reform.
"They have systematically at every turn blocked every single reform - they have blocked party funding reform, they have blocked reform on lobbying," he said.
"Believing any promises from them on political reform is a bit like accepting a consumer service guarantee from Del Boy - don't believe it, they are trying to treat you like fools."
For the Tories, shadow justice secretary Dominic Grieve said that only a new government could deliver real political reform.
"Labour have had 13 years to fix our broken politics, but all Gordon Brown can offer is partisan meddling with the electoral system and votes for teenagers. To get new politics we need a new government," he said.
In the Commons, Mr Cameron arrived for Prime Minister's Questions armed with the backing of another 30 business leaders for his promise to reverse part of the Government's planned increase in National Insurance contributions, due next April.
As Speaker John Bercow had to intervene repeatedly to enable the two leaders to be heard, Mr Cameron said that Labour was imposing a "jobs tax" which would kill off the recovery.
"This Prime Minister would wreck the recovery by putting a tax on every job, on everyone earning over £20,000, a tax on aspiration, a tax on every business in the country," he said.
Mr Brown, however, said that it was the Tories' plans to cut spending by £6 billion this year which would jeopardise the recovery and undermine frontline public services such as schools, hospitals and policing.
"To withdraw £6 billion from the recovery now would put jobs at risk, put businesses at risk, put growth at risk. We cannot cut our way to recovery but we could cut our way to double-dip recession," he said.
However, the Prime Minister came under fire from one of Mr Cameron's latest business backers after a television interview in which he repeated Lord Mandelson's claim that business had been "deceived" by the Tories.
Luke Johnson, founder of Risk Capital Partners, said: "I think that is insulting. I think businesses know very well what's going to encourage them to employ more people."Reuse content