Labour would be able to borrow £166 billion more than the coalition is planning under Ed Balls's new tax and spending rules, the Liberal Democrats have claimed.
A Treasury analysis of the shadow chancellor's fiscal rule, released by Lib Dem Treasury Chief Secretary Danny Alexander, suggested a Labour government would still be borrowing billions when the public finances would be back in balance under coalition plans.
But Labour said it was based on " made up numbers plucked out of the air".
And the Opposition said Mr Alexander appeared to have revealed spending plans for two years after those so far publicly announced by Chancellor George Osborne.
Mr Balls sought to rebuild Labour's reputation for managing the public finances with a binding commitment to run a surplus on the current budget and get national debt back on a downward path by 2020 at the latest.
However the Lib Dems said that would still allow him to borrow billions of pounds for investment at a time when, under the coalition's plans, investment would be financed from tax revenues.
Overall, according to the Treasury analysis, Labour would borrow £166.2 billion more than the coalition between 2016-17 and 2020-21. It calculates that Labour would still be borrowing £41 billion in 2019-20 and £32 billion in 2020-21 when planned coalition borrowing is down to zero.
Mr Alexander said: "This Treasury analysis shows that Labour have learnt nothing from the past and can't be trusted by the British people on the economy.
"Their new borrowing bombshell will pile another £166 billion of extra borrowing on to the debt mountain left by their catastrophic mismanagement of the UK economy.
"The Liberal Democrat plan to repair the economy is working with the right balance to get rid of the deficit, build a strong economy and deliver a fair society."
Mr Alexander ordered Treasury officials to carry out the analysis in response to a Freedom of Information request submitted by the Lib Dem MP Lorely Burt.
Shadow chief secretary Chris Leslie said: "Labour will deliver a surplus on the current budget and falling national debt as soon as possible in the next parliament, as we said last week.
"How fast we can go will depend on the state of the economy and public finances we inherit. But we do know that this Government is set to borrow £198 billion more than they planned because they choked off the recovery and flatlined our economy for three years.
"But the most revealing thing in Danny Alexander's press release is that he has announced, for the first time, coalition spending plans for 2019/20 and 2020/21.
"There are no spending plans or forecasts for those years in the Autumn Statement. Has Danny Alexander told his Lib Dem Cabinet colleagues that he has agreed another two years of spending plans with George Osborne?"
In a letter to Mr Balls, Chancellor George Osborne said the IFS analysis "confirms that you would borrow and spend more".
"It is vital that our political debate is based on the facts," he wrote.
"Our long-term economic plan is delivering growth and economic security for hard-working people.
"Despite your recent attempts to rebuild a reputation for fiscal responsibility, the truth that was buried in the small print is now clear: the Labour Party is the single biggest risk to the economic recovery."