Workers will be earning £11 less in 2015 than they did back in 2003

The figures show that after taking account of inflation, the median wage of a full-time employee will be the equivalent of 25,559 in 2015, less than the 25,570 earned in 2003.

The report, based on official government data, warns that Britain faces a "lost decade" for living standards.

Scottish economists described the figures as "too pessimistic" but warned that 2011 will be the toughest year since the banking collapse.

The latest figures have been released as a major 18-month investigation, The Commission on Living Standards, is launched today to look into how people are coping with the current economic challenges.

There is growing concern that the pressures now facing those on low-to-middle incomes are not just the result of the recent recession, but stem from deeper problems in the British economy.

Official figures show that wages stopped growing as far back as 2003, when the economy was still experiencing strong growth.

In the report, low-to-middle earners are defined as people living in households below median earnings but who are neither the poorest nor heavily dependent on benefits. This covers 11 million adults in six million households - earning between 12,000 and 30,000 for a couple with no children, and up to 48,000 for a couple with three children.

According to research, home-ownership is slipping out of reach of this group - with 41 per cent of young low-to-middle earners now in privately rented accommodation, compared with 14 per cent in 1988 - and a shifting jobs market means that mid-level jobs are increasingly scarce.

The commission will be chaired by Clive Cowdery, chairman of the Resolution Foundation, who said: "People on low-to-middle incomes are a third of the working population and play a critical role in our economy.It's now increasingly clear that the pressures they face - from flat wages to rising prices - stem from longer-term trends than the recent recession."

Labour leader Ed Miliband is today expected to echo the warnings of a "cost of living crisis" that will leave middle-earners struggling even if the economy recovers.

Mr Miliband is expected to say families with children will be worst affected, adding: "My fear for those on middle and low incomes is that more and more families will face a cost of living crisis that will see them left behind, even as the economy eventually recovers."

David Bell, professor of economics at Stirling University, said that although there were going to be "tough times ahead", he did not think the picture would be as bad as predicted.

"The median hourly wage in Scotland in 2003 was 8.94 and in 2010 it was 11.01, an increase of 30 per cent. The consumer price index increased over that period by 18 per cent, which means that the median Scottish wage had a real increase of about 12 per cent."

Dougie Adams, an independent economic consultant based in Edinburgh, and adviser to the Ernst & Young Scottish Item Club, said he thought that this year would be the toughest yet.

"We are at the mercy of global forces. Floods in Pakistan and Canada and drought in Russia have pushed food prices way up. Fuel is increasing and there are new taxation levels about to hit. Wages are not going up and employment is low.

"People talk about the banking crash year as being the worst but I think this year is going to be the worst yet. The good news is that after that I think that it will improve."

A spokesman for the Holyrood finance secretary said: "Scotland is being hamstrung by being in the Union."