UPDATE 1-Total's North Sea oil platforms hit by 24-hour strike

(Adds detail, context)

LONDON, July 23 (Reuters) - Around 40 rig workers started a 24-hour strike on Monday on the Alwyn, Elgin and Dunbar oil and gas platforms in the British North Sea, Britain's largest labor union said, but any supply disruption was expected to be mitigated by using oil in storage.

The strike began at 0500 GMT. Unite the Union and Total , which operates the platforms, earlier failed to agree over work shifts and pay, a union spokesman said.

The fields account for about 10 percent of Britain's gas output, while their oil production contributes about 45,000 to 50,000 barrels per day to the Forties and Brent Blend crude streams.

Forties and Brent Blend are key oil grades used to set the dated Brent benchmark that prices over half of the world's oil trades.

A Total spokeswoman confirmed that industrial action had gone ahead "despite the new proposals that were made by the company."

Early this month, Unite announced that its members working at the Total platforms would stop overtime and go on a series of 12- and 24-hour strikes starting on July 23.

The other 24-hour stoppages are slated to take place on Aug. 6 and 20, while the 12-hour ones will occur on July 30 and Aug. 13.

Total is seeking to extend the time workers at its fields spend offshore to three-week rotations instead of two to match the rota system at its newly acquired Maersk fields.

"However, we're pleased to be able to continue discussions with staff at the Shetland Gas Plant and remain committed to further consultation and open dialog with offshore staff," the spokeswoman said.

"What is at stake here is to ensure the long-term sustainability of our business in the North Sea, to enhance overall safety and remain the most efficient in our operations."

Other strikes are looming in the North Sea. Union workers at services company Aker Solutions announced their decision to strike at Equinor's non-producing Mariner platform, while 2,500 offshore oil service workers are being balloted for a possible separate strike.

Last week, 1,600 Norwegian rig drillers ended industrial action. (Reporting by Shadia Nasralla, Ron Bousso and Julia Payne; Editing by Dale Hudson and Louise Heavens)