Drivers of London's underground train system went on strike Sunday in a dispute over holiday pay that is expected to affect the busy holiday shopping season.

ASLEF, a trade union for the drivers, had asked for triple pay and a day off for drivers working on December 26, a national holiday.

"Drivers felt they shouldn't have to work day after Christmas, and if they do, they should get the extra pay," said Chris Proctor, a spokesman for the trade union.

Transport officials turned down the request. Boxing Day falls on Sunday, which means the holiday will be on Monday and drivers can get extra pay then, the transport officials said. Striking drivers plan to return to work Monday.

"The completely unnecessary strike action is likely to cause significant disruption, but London Underground is working to ensure as many services as possible will operate," London Underground officials said in a statement. "London's 700 bus routes and some river services will be in operation."

Unions signed an agreement that gives them increased leave time and pay on condition they work some holidays, including Boxing Day, London Underground said.