Network Rail has served papers on the Rail, Maritime and Transport union, "calling into question the validity" of their strike ballot.

Last night furious Bob Crow, RMT general secretary, raged: "We will be mounting the most robust defence possible in the High Court against this attack by Network Rail. This is a scandalous attempt by Network Rail to use the full weight of the anti-union laws to deny our members their basic human right to withdraw their labour and we will fight this assault on our union to the hilt."

The union says 12,000 maintenance members voted by 77% in favour of strikes - the first for 16 years - while its 6,000 signallers backed industrial action by 54%. But the firm will claim the union's figures are "inaccurate".

Four days of strikes have been announced from next Tuesday. Unions say it is in protest at plans to axe 1,500 jobs and overtime cuts. Passengers face "severe" disruption if the action goes ahead.

Last night a company spokesman said: "Talks continue and our aim is a negotiated settlement, but we must explore all avenues at our disposal and that includes legal ones. Papers have been served on the RMT."