The Rail Maritime and Transport union said the dispute at the East Midlands company centred on plans to use managers to guard trains on Sundays as well as a row over conditions.

General secretary Bob Crow said: "The company is acting outside the agreed negotiating machinery and has already poured petrol on the flames by stopping RMT reps being released to undertake normal union duties."

And he added: "We want a negotiated settlement to this dispute, but that will not happen while the company is throwing its weight about, and the union is urging its members to return a solid vote for strike action."

East Midlands Trains managing director Tim Shoveller said: "From the beginning of our franchise we have actually run over 97% of all Sunday services. Under the previous franchise, on some Sundays, there were as few as 50% of services running because of a shortage of staff.

"This is a fantastic improvement, achieved through better management and with the flexibility and commitment of our staff.

"I thank our staff for their continued dedication, but we must now find a permanent solution to ensure the reliability of Sunday services for our passengers ongoing.

"We're confused and disappointed that the RMT do not understand that as a train operator we must be confident that we have enough senior conductors available to operate our trains every day of the week for passengers, it's not something we can leave to chance."

The Derby-based company took over the franchise at the end of last year.