Royal Navy submarine HMS Tireless has arrived in the southern Indian Ocean to help in the search for missing Malaysia Airlines flight MH370.
The Trafalgar Class submarine arrived yesterday in the area - the focus of the search for the missing Boeing 777 - where it will help in the hunt for the plane's flight recorders.
Flight MH370, bound for Beijing from Kuala Lumpur, disappeared on March 8 with 239 people on board, with a three-week search finding no sign of it.
The search area, which has repeatedly shifted as experts analyse radar and satellite data, is currently focused on a 98,000-square mile area off Perth, Australia.
Royal Navy survey ship HMS Echo is due to begin search efforts tomorrow, but a source said that Defence Secretary Philip Hammond has called his Malaysian counterpart to say that HMS Tireless has arrived in the area, where it is searching for the flight recorders.
The source said: "In addition to the deployment of HMS Echo, a Royal Navy survey ship, which begins search efforts tomorrow, a Royal Navy Trafalgar-Class submarine has recently arrived in the area and is conducting search operations for the flight recorders.
"She was ordered to move from an operational tasking to the search area around a week ago and arrived on station yesterday.
"HMS Tireless holds advanced search capabilities, but the task in hand remains a tall order and the search area is immense. Her deployment is being co-ordinated closely with our Australian and US colleagues.
"The Defence Secretary informed his Malaysian counterpart of the additional British contribution during a phone call this evening."
HMS Tireless has advanced underwater search capability which it is hoped will help in the hunt for the missing jet.
It is just one part of the UK's contribution to the search, with HMS Echo due to arrive in the southern Indian Ocean to help in the hunt for the plane's black box, as well as helping in the search for debris on the surface.
Britain is also helping with technical assistance and specialist capabilities from across the Ministry of Defence, the Hydrographic Office, Department for Transport and the Met Office to help try to find flight MH370.
Four RAF personnel on secondment to the Royal Australian Air Force and Royal New Zealand Air Forces are also actively involved in maritime search operations.
A Ministry of Defence spokesman said: "While we do not routinely comment on submarine operations, we can, exceptionally, confirm that HMS Tireless has been tasked to assist in the humanitarian search mission for Flight MH370.
"We do not comment on the operational capabilities of our submarines."
Earlier, the Malaysian government said investigators are conducting a forensic examination of the final recorded conversation between ground control and the cockpit, after it changed its account of the final voice transmission from the missing plane.
The government had transcribed the final voice transmission received by ground controllers at Kuala Lumpur's international airport as "All right, good night", but today released a transcript of the conversation which noted the final words as: "Good night Malaysian three-seven-zero."
Australian prime minister Tony Abbott has said that although the search for the aircraft has been slow, difficult and frustrating, it will continue indefinitely.
He said the intensity and magnitude of operations "is increasing, not decreasing", and added: "If this mystery is solvable, we will solve it."
International Air Transport Association (Iata) director general Tony Tyler said there is "disbelief" that flight MH370 "could simply disappear" and a plane must "never again go missing in this way".
He said the incident has highlighted two areas where improvements were needed - aircraft tracking and passenger data.
Speaking at an Iata conference in Kuala Lumpur, he said: "In a world where our every move seems to be tracked, there is disbelief both that an aircraft could simply disappear and that the flight data and cockpit voice recorders are so difficult to recover."
He said that the Air France 447 incident, when a plane crashed into the Atlantic in June 2009 claiming 228 lives, had "brought similar issues to light a few years ago and some progress was made".
He said: "But that must be accelerated. We cannot let another aircraft simply vanish.
"Speculation will not make flying any safer. We should not jump to any conclusions on probable cause before the investigation into MH370 closes.
"There are, however, at least two areas of process - aircraft tracking and passenger data - where there are clearly challenges that need to be overcome."
On passenger data, he said: "It is important to remember that airlines are not border guards or policemen. The checking of passports is the well-established responsibility of governments.
"The industry goes to great effort and expense to ensure that governments have reliable information about passengers before an aircraft takes off. Governments need to review their processes for vetting and using this data, such as Interpol's stolen and lost passport database. This information is critical and it must be used effectively."
Mr Tyler added: "In 2013, there were over 29 million flights operated on Western-built jet aircraft, with 12 hull losses. That is one accident for every 2.4 million flights and a 14.6% improvement on the five-year industry average.
"Accidents are rare, but the current search for MH370 is a reminder that we can never be complacent on safety. It may well be a long time before we know exactly what happened on that flight.
"But it is already clear that we must never let another aircraft go missing in this way. And it is equally clear that governments must make better use of the passenger data that they mandate airlines to provide."
Nuclear-powered submarine HMS Tireless was launched in 1984.
In 2011 the submarine spent 10 months on deployment in the Middle East, where it conducted covert missions to support counter-terrorism, counter narcotics, and counter-piracy operations.