A shareholders action group has threatened to sue the government over its decision to put Railtrack into administration.The Railtrack Private Shareholders Action Group, which has about 45,000 members, confirmed yesterday that it expected to issue a writ against Alistair Darling later this week.

The lawsuit will accuse the transport secretary of "malfeasance" over his department's decision to put Railtrack into administration and replace it with Network Rail.

"We await the details of the case. If proceedings are brought, the department will defend them robustly," said a Department for Transport spokesman.