LONDON — Engine failures on Britain's Type 45 destroyers have been substantially reduced, but a final solution to the problems awaits implementation of a yet to be approved power improvement plan, Defence Secretary Michael Fallon said in a letter to the parliamentary Defence Committee.

Fallon declined to publicly detail the extent of the problem with the destroyers' power plant but said in the letter March 23 that remedial action to date meant that: "In broad terms, current failure rates are one third of those experienced in 2010."

Fallon was responding to a letter from Defense Committee Chairman Julian Lewis, who requested an explanation after concerns were raised in Parliament and elsewhere over the reliability of the integrated full electric propulsion system (IFEP) used on the Royal Navy's most powerful warships.

The Ministry of Defence is having to put millions of pounds aside to fix the problem on the almost-new warships. The solution includes installing an additional diesel engine to the two gas turbines and two diesels already fitted.

The six-strong fleet of destroyers built by BAE Systems has suffered a catalog of engine problems, most publicly in 2009 when the recently commissioned first-of-class HMS Daring lost power in the Atlantic Ocean.

The BBC reported in January it had seen an email from a serving Royal Navy officer saying that "total electric failures are common" on the destroyers earmarked to help protect Britain's new 65,000-ton aircraft carriers when they enter service later in the decade.

The Type 45s were the first complex warships to employ an IFEP system, which uses gas turbines and diesels to power electric motors, which turn the propellers.

Problems with the innovative system emerged during shore testing in 2005 and have been denting reliability of the destroyers' power system ever since.

The Type 45s are powered by two Rolls-Royce WR21 gas turbines and two Wartsila supplied diesels.

Rolls-Royce secured the power plant deal in partnership with Northrop Grumman in a competition with General Electric's LM2500.

A Rolls-Royce spokesman said: "We continue to work alongside industry partners on making improvements to the propulsion system."

Part of the WR21 package is an intercooler recuperator, which recovers exhaust and recycles the gas into the engine, improving fuel efficiency.

The website for Save the Royal Navy reports the intercooler unit has a design flaw that occasionally causes the WR21s to fail.

"When this happens the electrical load on the diesel generators can become too great and they trip out leaving the ship with no source of power," according to the website.

Fallon's letter said a study in 2011 established there was no single cause for the low reliability but rather a "large group of unconnected individual causes."

The MoD and contractors have been working to resolve the reliability issue for years.

A series of enhancements have been implemented across the fleet that have helped reduce the scale of the reliability technical issues by two-thirds.

But, Fallon said, "we know now that reliability improvements alone will not enable us to realise the potential of the class [of warship]. Rather, the performance and design of the power and propulsion system is simply not able to deliver the resilience and reliability required."

Part of the solution is to add another diesel engine.

"A power improvement plan (PIP) will improve system resilience by adding upgraded diesel generators to provide the electrical generation capacity required to meet many propulsion and power requirements without reliance on WR21," the letter said.

The MoD is talking with four contractors to assess the technical options, and the letter said an assessment phase would likely be launched later this year to consider detailed technical proposals ahead of seeking approval to proceed with the program.

Fallon said the cost and implementation timetable would not be known until the final design solution has been selected.

The work is likely to be built into major refit programs planned for the destroyers starting at the end of the decade.