By Rhys Jones
LONDON (Reuters) - Britain has deferred to 2015 a firm commitment on how many Lockheed Martin Corp F-35 Joint Strike Fighter jets it will buy, adding to uncertainties over the multinational program which has recently been questioned in the U.S. Congress.
"We will not make final decisions on the overall number of aircraft we will order before the next planned Strategic Defence Review (in 2015)," a Ministry of Defence MoD spokeswoman said on Tuesday, adding an initial order would be placed next year.
The F-35 project ranks as the most expensive U.S. arms program but has been criticized for cost overruns at a time when next week's U.S. fiscal 2013 budget plan is expected to postpone funding for 179 warplanes until after 2017 -- a move that has prompted international partners to question their own procurement plans.
Britain in 2001 committed to buy 138 of the multirole stealth aircraft, but the current coalition government in its 2010 defense review said it would cut the number of F-35s it had on order without saying by how many.
Britain has so far placed a firm order with Lockheed for three F-35 test and evaluation aircraft costing $632 million.
A spokesman for Lockheed, the top U.S. defense contractor, said Britain's total order had not been revised down and remained at 138. Britain was due to receive its first F-35 in June.
Australia has said it is rethinking its plan to buy 12 of the radar-evading jets, while Turkey has put off buying two jets and Italy may follow suit, according to FlightGlobal.
Other partners in the project include Canada, Denmark, the Netherlands and Norway.
While there have been reports Britain will cut its order to 50 F-35s, the MoD said it did not recognize that figure.
Expectations for the number of F-35s Britain will eventually order have been curtailed since the MoD's decision to use only one aircraft carrier, which will routinely have 12 fast jets embarked for operations, while retaining a capacity to deploy up to 36.
In the United States, cuts to the F-35 program are part of the Pentagon's plan to start implementing $487 billion in defense spending reductions over the next decade.
(Additional reporting by Mohammed Abbas; Editing by David Holmes)