A total of 12 companies are affected

The strikes, beginning on 6 and 27 May, will both immediately follow bank holiday weekends.

There been three 24-hour stoppages in recent weeks, with the last taking place just before the Easter weekend, affecting both commuters and holidaymakers.

Worst hit were Wales & Borders, Central Trains and Wessex Trains but other companies were able to operate almost normally.

The dispute centres on a change to the safety "rule book" whereby the driver now protects the train while the guard is responsible for passenger safety.

Previously both driver and guard were responsible for putting warnings on the line if there was a breakdown or accident.

Rail bosses are furious at the strike action with Scotrail boss Peter Cotton saying it was a "needless, pointless dispute" about safety rules.

'Disappointed'

Bob Crow, general secretary of the RMT, said: "The employers know that it will cost them nothing to settle this dispute but they have made no effort whatsoever to come to an agreement.

"This is a dispute about safety pure and simple. The guards' role has been watered down and it needs to be restored to the rule book in full.

"Our members and the travelling public have the right to know that the safest working practices are in place on the railways."

A spokeswoman for the Association of Train Operating Companies, which represents the rail firms affected by the dispute, said: "We are disappointed by the threat of more industrial action in what we consider to be a pointless strike.

"We are confident that train services will still run despite the strikes. There is a proper way of resolving this dispute and we urge the union not to take industrial action."

The train companies affected by the strikes are Arriva Trains Merseyside, Central Trains, Connex South Eastern, South Central, Midland Mainline, ScotRail, Silverlink, Thames Trains, Virgin West Coast, Virgin Cross Country, Wales and Border and Wessex Trains.