An early warning detection system known as Crowsnest for Britain's new aircraft carriers will be ready when the ships come into service rather than some time in 2020, the Defence Secretary has said.

Philip Hammond told MPs the system had been brought forward after management of the equipment budget freed up the money.

Earlier announcements by the Ministry of Defence indicated the system, based on the Merlin helicopter and designed to provide state of the art surveillance around the carriers, would only go into trials in 2020.

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The first of the two Queen Elizabeth-class aircraft carriers is due to begin sea trials in 2017, and come into service in 2020.

Speaking during Commons questions today, Mr Hammond said: "I'm glad to be able to say we have been able to advance the Crowsnest airborne early warning capability project as a result of prudent management of the MoD's equipment programme.

"We will have the full operating capability available when the aircraft carriers go into service."

In a letter to defence select committee chairman James Arbuthnot in January, last year, former defence minister Andrew Robathan said ­mitigation measures based on the ships themselves or helicopters had been planned.