Unite the union has raised concerns over the defence secretary's decision to buy American without competitive tendering.
The union said it was worried about the lack of "offset agreements" in the proposed Boeing deal, work that could be earmarked for British defence firms to safeguard skilled jobs.
The deal is to buy nine Boeing Poseidon P-8A patrol aircraft, estimated to be worth £2.2bn, is expected to be signed at next month's Farnborough air show, the union said.
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Unite assistant general secretary for manufacturing, Tony Burke said: "At a time of increased international tension and security concerns, defence secretary Michael Fallon needs to come clean with the British public on the secretive nature of the UK’s arms procurement policy.
"As a first step, the government needs to ensure that a substantial amount of P-8A production work is undertaken in the UK, with all the support work to maintain these aircraft in the years ahead. Apparently, the P-8A will not be using UK weapons, which is a disgrace.
"Michael Fallon needs to make a statement to MPs before any contracts are signed and before Parliament rises for the summer recess on 21 July on what is happening specifically with the replacement for Nimrod and also regarding the Apache helicopters’ order."
The new generation of planes is to bulk up the UK's maritime surveillance capabilities.
The plans began in November last year, when the government said in its national security and defence review: "Boeing P8 Maritime patrol aircraft to increase further the protection of our nuclear deterrent and our new aircraft carriers. These aircraft will be based in Scotland and will also have an overland surveillance capability".
Unite was further incensed as it comes after reports that Boeing could receive as many as 50 Apache helicopters, rather than Yeovil-based AgustaWestland.
The Ministry of Defence was not available for comment at the time of writing.