A leading train company has told union leaders it would not renegotiate a pay deal as workers prepared to vote in a strike ballot in a row over an Olympic Games bonus.

The Rail Maritime and Transport union (RMT) said it wants more money for its members at South West Trains (SWT), pointing out that other transport firms were paying a bonus to cover the period of the Games.

Union members will start voting next week on whether to take industrial action.

SWT warned the RMT that it will not be renegotiating the pay deal it said was agreed four months ago which already covers the period of the Games, accusing the union of reneging on the agreement.

A "significant" pay increase of 4.75% was agreed in February, specifically including arrangements during the Games period, said SWT.

A spokesman said: "We are disappointed the union has decided to press ahead with this unnecessary ballot. We negotiated a deal with the RMT in good faith and we have stuck to it. We want the union to be 100% clear about our position: we will not be renegotiating our agreement that already covers Olympic arrangements."

Bob Crow, general secretary of the RMT said: "South West Trains have now resorted to sheer bullying of the workforce in their desperate attempts to force our members to work longer and harder during the Olympics for nothing in return while the company makes a killing for its shareholders from the increased Olympics services.

"RMT is campaigning for a massive yes vote for action in this ballot to force the company to see sense and to demonstrate the level of anger this issue has generated across the service. Other train operators throughout the London area have shown that they are prepared to play fair with their staff over the Olympics.

"Instead of resorting to threats and bullying, SW Trains should do the same."

London bus workers are planning two strikes next month in pursuit of a £500 Olympic payment.