The Department for Transport believes the charges are uncompetitive, and has referred the matter to the Office of Rail Regulation.

The department has been holding talks since early 2005 with the rolling stock operating companies (Roscos).

The firms lease trains and carriages to rail operators for about £1bn a year.

The department has asked the Office of Rail Regulation to consider "a further referral to the Competition Commission for a market investigation".

'Excessive charges'

The largest train rolling stock firm - Angel Trains - is owned by the Royal Bank of Scotland, and has some 5,000 assets.

The other two are HSBC Rail, owned by HSBC, and Porterbrook, owned by the Abbey Group.

"Those discussions have progressed but it has become necessary to bring the matter to a firm conclusion," said a statement from the Department for Transport.

"The department argues the excessive leasing charges are the result of a deficiency in the market created by privatisation that should be remedied."

It said that a market investigation reference under the Enterprise Act was the "appropriate mechanism for dealing with such an issue".

However, former rail regulator Tom Winsor, who is now an adviser to Angel, said the department's investigation was based on "out-of-date" data, and that its action was an "unjustified hit" on the rolling stock firms.

There are about 12,500 trains and carriages on lease, with roughly 60% pre-dating the privatisation of British Rail in the mid-1990s.