The rail network is set to be hit by two strikes by signal workers, threatening chaos for passengers, after a fresh round of pay talks broke down.
Unions were originally going to strike during the World Cup
The Rail Maritime and Transport Union said thousands of signal workers will walk out for 24 hours from noon on Friday 21 July.
A further 48-hour stoppage will start at noon on Thursday 27 July, it said.
Network Rail said it had worked hard to avert industrial action over a 21-month pay deal at the centre of the dispute.
RMT general secretary Bob Crow said the union remained available to resolve the issue but added he could not understand Network Rail's refusal to consider a union request of a one-year deal.
"Network Rail has just agreed a one-year deal for over 15,000 infrastructure workers but is refusing to give 5,000 signallers the same - that does not make sense," he said.
"The company is fixated with a 21-month deal but all it has to do is apply the figures in the offer relating to the first year and it will be acceptable to the union."
'End of the line'
Network Rail Chief Executive, John Armitt, said it urged the RMT to take the offer as it was the "end of the line for negotiations".
"We are extremely disappointed by the RMT's decision, which means we are now facing a rail strike that will cause disruption for millions of people," he said.
Network Rail said the company was unable to meet the union's demand for a further substantial increase on top of the £2,500-per-employee raise already on offer.
An estimated 5,000 rail workers had planned to strike twice in June during earlier pay negotiations.
Both dates clashed with England World Cup group matches, but the union eventually called the strikes off so staff could be consulted over the new pay offer.