A defence minister has insisted it would be "quite wrong" to predict when a new fleet of maritime patrol aircraft (MPA) will be fully operational.
Conservative frontbencher Philip Dunne told the Commons a letter has been submitted to the United States as part of work to buy nine Boeing P8 aircraft.
But Mr Dunne said he could not pre-empt negotiations and would be unable to state when all the aircraft will be available until these talks have concluded.
The MPA fleet will be based at RAF Lossiemouth in Scotland and will be fitted with a range of sensors to hunt enemy submarines and ships and be able to carry torpedoes - filling a gap left by the 2010 decision to scrap a new generation of Nimrod aircraft.
SNP Westminster leader Angus Robertson pressed the UK Government to reveal when the entire fleet will be up and running in his Moray constituency and for training to also take place at RAF Lossiemouth.
Mr Robertson said his party welcomed the recent "U-turn" as it believes it is "negligent and dangerous" for the UK not to have MPA.
He asked Mr Dunne: "Can you confirm to the House when the entire fleet will be operational?"
Mr Dunne replied: "What we made clear in the SDSR (Strategic Defence and Security Review) is we will be procuring nine P8 maritime patrol aircraft and the fleet will be procured through an FMS procurement contract, the letter for which has already been submitted to the United States.
"The first aircraft will be operational in 2019."
In his second question, Mr Robertson said to Mr Dunne: "People will note you weren't able to answer the question of when will the entire fleet be operational, so perhaps when you come back after my second question you will answer the first.
"The RAF is currently maintaining its skill base by training on maritime patrol aircraft with the United States, with Canada, with Australia and New Zealand.
"So do you acknowledge the importance of MPA training, which was scheduled to be based at RAF Kinloss before the scrapping of the Nimrod fleet?
"Will the Government ensure that training for P8 MPA aircraft is based at RAF Lossiemouth, as it currently is for both Tornados and Typhoon?"
Mr Dunne replied: "As we are currently in contractual negotiations for the procurement, it'd be quite wrong of me to pre-empt precisely the nature of those negotiations so I can't answer your initial question as to how many aircraft will be available by when until such time as the contract has been concluded.
"As to training, you're quite right to reflect the fact we have crews in service on this platform with other users in the United States and the training basing will be established as part of the procurement process in the coming months."
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