The Catalyst Forum said savings would be at least �500m a year

The Catalyst Forum said most passenger services could be taken back into state hands without further cost by 2012.

"We need a railway that is public owned and publicly accountable, and which delivers a real return on the public money invested in it," Catalyst said.

The report came as the RMT union promoted a run of marches to press MPs to return railways to public control.

'Financial hole'

The marches will start in Glasgow on 16 April, ending in London on 30 April with a large rally, putting pressure on the government ahead of the election.

RMT general secretary Bob Crowe said: "Catalyst have shown that public ownership will plug an enormous financial hole in the railways that has been leaking at least �800m in public money into private pockets every year since 1996.

"The study shows that privatisation has already cost taxpayers more than �6bn, and that bringing the railways back into the public sector will reap a rail rebate for Britain of at least �500m every year," he said.

Any private sector investment in the railway must ultimately be paid for by fare payers and taxpayers - with interest Catalyst Forum report

Catalyst's Martin McIvor said: "Ministers have suggested that we cannot afford to take the railways back into public ownership. The reality is that we cannot afford not to."

The Catalyst Forum report said "the most conservative estimates" indicated public ownership would produce immediate savings of �500m a year.

"Any private sector investment in the railway must ultimately be paid for by fare payers and taxpayers - with interest."

The Department for Transport said it believed a public private partnership (PPP) was still the best way of financing the railways.

'Pointless'

A spokesman said the government was financing the rail network to the tune of �17m a week and that "private investment matched that".

"It's not just the matter of the cost of buying back all the trains and the track when you renationalise, it is the fact you lose all that investment," he said.

A spokesman from the Association of Train Operating Companies (ATOC) said: "What passengers want is for the railways to deliver for them today and tomorrow.

"Today's railway is doing just that: punctuality is improving; thousands of new trains are entering service; and the quality of customer information has improved unrecognisably over the past 10 years.

"Trying to hark back to yesteryear is pointless, and irrelevant to the needs of today's passengers."