Labour will attempt to lead a cross-party rebellion to enforce a new £2bn tax on bankers to fund a back-to-work scheme for jobless young people, the shadow chancellor, Ed Balls, said on Sunday.

Balls is calling on rebel Tory and Lib Dem MPs to back the plan – an extension of a Labour tax on bankers' bonuses – in a vote in the Commons later this month. It would provide £1.2bn for a house-building scheme to provide low-cost homes and create 20,000 new jobs, £600m in payments to employers to take 90,000 more under-25s and £200m for unemployment "blackspots" around the country.

It comes as a group of leading economists makes a major intervention, warning that Britain needs a plan B as an alternative to the coalition's spending cuts, in case the economy becomes too fragile to withstand the shock of the deficit-reduction programme.

Those voicing concerns include two ex-senior government economists and two more who previously signed a high-profile letter last year back the Tory-led coalition's plans.

Labour has made youth unemployment a priority in opposition. Balls, writing in the News of the World, said: "Putting young people on the dole is not just a waste of talent but a waste of money too. And failing to get Britain back to work fast enough is helping to push up the benefits bill by over £12bn – that's £500 per household.

"That's why this week the shadow work secretary Liam Byrne and I will launch a new campaign for a £2bn tax on bankers' bonuses which should be used to create 100,000 more jobs for young people, build more affordable homes and support small businesses.

"Our plan will be put to a vote in parliament – and we're asking MPs from other parties to back it."

He also told Sky News: "We were told in the autumn by the chancellor and David Cameron that it would work and we are out of the danger zone and by cutting this fast ... the private sector would be spending and investing more, confidence would rise, the economy would do well. I'm afraid the opposite is happening. Confidence is down and we're now seeing week by week more evidence that the economy is stalling."

However, the foreign secretary, William Hague, said that the government was right to stick to its course. "The government strategy is endorsed by the IMF, it is endorsed by the OECD, the G20, by all the major business organisations in this country and the harsh truth is that Gordon Brown did not leave this country with the luxury of a plan B or a different economic strategy," he said.

"We have to get down the debts that he left, control the deficits that he left and if we wavered from that for a moment then economic confidence would be reduced, the confidence of the financial markets would be very severely affected, so it is vital to continue on the course that we've cut."

He added: "We've seen what is happening in Greece and Portugal and the last government left us with the same level of deficit as Greece and Portugal."