DML has a workforce of about 4,800

White-collar workers were leaving their shifts three and a half hours early throughout Friday in the first of a series of balloted stoppages.

The Prospect union said it followed management's decision to impose a new pay performance structure.

A spokesman for DML, which runs the dockyard, said site and operational safety were not being affected.

Unions said the action had caused major disruption to work at the yard.

Safety unaffected

The action follows months of negotiations after DML's decision to impose a new pay structure at the yard which reserves pay progression payments solely for good or excellent performers.

The previous progression system was unilaterally terminated by the company in 2004, unions claimed.

The Prospect union said 94% of the 1,400 members balloted voted in favour of taking part in a stoppage.

Engineering group Babcock International bought the Devonport dockyard in a �350m deal earlier this year.

Devonport maintains, upgrades and fuels the Royal Navy's submarines. It has a workforce of about 4,800.