Security has been stepped up at Devonport naval base after peace campaigners broke in and boarded a nuclear submarine.

The move came as a report by the Ministry of Defence blamed poor execution of security measures for the embarrassing incursion.

Restricted access to dock gates and a clampdown on public tours of the base are part of the new measures. Other details are not being released.

Two supporters of the anti-nuclear campaign group Trident Ploughshares were arrested on 15 November after boarding Trident sub HMS Vanguard.

Traders protest

An MoD spokeswoman said the investigation had found that proper security measures were in place prior to a week of demonstrations at the base by Trident Ploughshares.

But those measures were not followed through and now there were additional security measures.

She said: "We are working hard to ensure such a breach of security doesn't happen again."

The closure of the gates has caused objections from local traders who have been hit by a slump in trade from base workers.

The MoD said: "We gave great sympathy for them. The closure of the gates has had quite an impact, but the priority is to get the security right."

Protest at Devonport has been peaceful

The two demonstrators said at the time of their arrest that they had entered the base wearing ordinary clothing.

Elisa Silvennoinen said: "We walked inside the base and didn't see anybody.

"We got on to the nuclear submarine and walked around it first and then pushed the fire alarm."

They would not reveal where their point of entry was, or how they by-passed the security system.

Ms Silvennoinen and Petter Joelson have been charged with criminal damage and are due to appear before Plymouth magistrates next month for a pre-trial hearing.

HMS Vanguard arrived at Devonport in February this year for a two-year refit and refuelling.

It is the first of the Vanguard class submarines to be refitted at the naval base and is one of seven nuclear-powered submarines based there.