AirTanker planes are encountering leakage problems during in-flight test refuelling of British Tornado jets and face compensation claims from Britain's Ministry of Defence (MoD) if the problems result in delays or extra costs.

"As would be expected with a new aircraft, there have been some technical problems, but these are being addressed," AirTanker said in a statement, adding that it still expects to deliver a core fleet of nine aircraft by 2014.

News of the leaks raise a fresh question mark over a contract that has been dogged by controversy since the MoD agreed a 10.5 billion pounds ($16.7 billion) deal to lease 14 of the modified Airbus A330 aircraft from the EADS-led AirTanker consortium in 2008.

Nine of the planes due to enter service in 2014.

"If the consortium fails to meet its obligations the MoD will expect to recover any additional costs incurred," an MoD spokeswoman told Reuters.

A source close to AirTanker said the problem was in pipes which connect the Voyager to Royal Air Force (RAF) Tornado warplanes which leaked when fuel was pumped through them during mid-air testing. The source said the refuelling trial was continuing.

The tanker planes, which are 60 metres long, can carry 100,000 litres of fuel and pass it on to other planes at a rate of 5,000 litres a minute.

The MoD was criticised two years ago after it was revealed that Voyagers lacked the protective gear necessary to operate in war zones.

The Voyager is due to enter service for training later this year, allowing flight crews to train on the aircraft for aeromedical and transport roles.

In 2010 Parliament's Public Accounts Committee concluded in a report there were "significant shortcomings" in the way the ministry had contracted to lease the tankers.

However, the government pushed ahead with the deal because the RAF urgently needs replacements for its ageing Lockheed Martin TriStars and Vickers VC10s.

The AirTanker consortium also includes Britain's Cobham , the pioneer of in-flight refuelling, Rolls-Royce and Babcock Internatioanl, as well as France's Thales .

Britain's Sun newspaper on Thursday reported that the Voyager had proved to be compatible with U.S. jets.