THE fourth Astute-class submarine - Audacious - is 10 months behind an already-delayed completion date, according to a government report.

The cost of building four new attack submarines jumped by almost £200m last year and the date for the next boat entering service has been delayed by 10 months, according to a Ministry of Defence report.

The fourth Astute boat, HMS Audacious, was initially due to enter service in August 2015 but this had already slipped to next January.

The MoD report reveals the service date has been delayed another 10 months, to November 2018.

However, one shipyard source told the Evening Mail that the delay was understood to be four months because of late deliveries of electrical components and materials. It is hoped that the delay can be shortened.

A document published by the Ministry of Defence (MoD) last month revealed that the costs of some of Britain’s biggest defence equipment projects soared in 2015-16, compared to an overall fall the previous year, the Sunday Times has reported.

The biggest annual cost rise last year — totalling £249m — was in a £1.6bn programme to build nuclear reactors for submarines and redevelop the Rolls-Royce facility in Derby.

The project to build another four Astute submarines increased by £191m last year. Three boats — Astute, Ambush and Artful — have already been delivered and are replacing the Trafalgar boats as they are gradually withdrawn from service.

A spokeswoman for BAE in Barrow said: "The design and build of a nuclear powered submarine is one of the world's most complex engineering challenges.

"BAE Systems is committed to working with the UK MOD, its industry partners and the ‎wider submarine enterprise to deliver these key national programmes.”

Rolls-Royce declined to comment specifically on the programme but said the company had a “robust programme in place to deliver continuous operational efficiencies”.