Labour challenges Osborne over pre-election scrutiny
A change in the law to allow parties' tax and spending commitments to be independently audited would help ensure an "informed debate" on the economy at the next election, Labour has said.
The opposition wants the Office for Budget Responsibility's remit to be revised to allow it to assess whether parties' manifesto pledges add up.
It is challenging Chancellor George Osborne to back the "important reform".
Ministers say the move is a "stunt" to distract from Labour's own plans.
The OBR was established in 2010 to provide independent forecasts of economic growth and borrowing.
The watchdog's charter states that it has a "broad remit" to analyse the government's performance against its fiscal targets and to examine and report on the sustainability of the public finances.
One of its functions is to scrutinise the Treasury's costing of tax and welfare spending measures in the run-up to each year's Budget announcement and the Autumn Statement.
Labour is calling for the OBR's role to be "enhanced" to allow it to run the rule over parties' economic programmes ahead of the next election - scheduled for May 2015.
Shadow Chancellor Ed Balls put forward the idea at last month's party conference.
He has now written to his Conservative counterpart to urge him to back a change to the law, without which OBR officials have said they would not be legally able to undertake the work.
Under Labour's proposals, the OBR would be authorised to scrutinise and certify the "policy costings" of any party with more than 5% of MPs in Parliament, if it receives sufficient information to do so.
The watchdog would be expected to say "whether it agrees or disagrees" with the numbers and publish its findings.
In his letter to Mr Osborne, Mr Balls said he believed the watchdog would be able to fulfil the role without commenting on individual policies or "being drawn into party politics".
He suggested Labour may put forward an amendment to next year's Budget bill to propose the change but only if it could build cross-party support for the move.
"I hope you will support this important reform, which I believe will enhance the role of the OBR while maintaining its impartiality and independence and ensure a more informed debate in Britain at the next election," he said.
The head of the OBR, Robert Chote, has said it is up to Parliament to decide the watchdog's powers but insisted it would need more resources to be able to do the work.
He also warned that it would have major implications for the run-up to the election, with parties having to publish their manifestos much earlier than they traditionally do to allow them to be analysed.
The Conservatives have said the OBR's remit is clearly defined and was agreed by Parliament on a cross-party basis when it approved legislation setting up the body.
But Andrew Tyrie, the Conservative MP who chairs the Treasury Select Committee, said the idea should be considered as part of an undertaking to give the OBR "absolute discretion" in its work.
"If the OBR were to undertake this work, it would be essential to obtain a measure of cross-party support about the terms under which it would be conducted," he added.
The Conservatives have accused their opponents of making a series of unfunded policy commitments which would increase the deficit and debt levels.
Labour have said they would stick to the outline of the government's spending plans for 2015-16 and promised an "iron discipline" on spending if they are re-elected.