Campaigners fear the Tories are set to snub British workers and award a £1.6billion defence deal to a Spanish-led bid.
The Government is poised to announce the winner of a competition to build three, 40,000-tonne Fleet Solid Support ships to resupply Royal Navy vessels at sea.
A decision is expected within weeks - and if the contract goes to a consortium spearheaded by a company based abroad it will be the first time British warships have been built overseas.
Industry insiders believe Team Resolute, led by Spanish shipbuilder Navantia, could land the deal - scuppering the hopes of Team UK, which includes British firms such as BAE Systems and Babcock.
Team UK estimates 2,000 British jobs would be safeguarded directly by the programme if it scoops the contract, with 1,500 more protected in the wider supply chain - and another 2,500 jobs indirectly benefiting in communities surrounding the yards.
But insiders warn that if Team Resolute is picked, up to 40% of the work may go overseas.
Defence Secretary Ben Wallace is set to be grilled about the plans when he appears before MPs on Wednesday.
Prospect union general secretary Mike Clancy said: “At a time of economic turmoil and deteriorating global security, building warships overseas for the first time in UK naval history would be a calamity.
“We have the opportunity to invest in UK skills and support our dockyards by spending this money here – why you would want to spend it boosting Spain’s economy instead of our own is beyond me.
“The new Prime Minister has indicated that stimulating growth is one of his main priorities after securing the public finances.
“With a meaningful boost to the economy and a significant return to the Exchequer if these are built in the UK, this procurement decision really ought to be an easy one.”
Labour MP Kevan Jones, a former Defence Minister who chairs the All-Party Parliamentary Group on Shipbuilding, said: “The UK is the only G7 country to offshore warship production - no other G7 country would even consider such a policy.
“The Government must now look at the effects of its offshoring policy on weakening national resilience and the loss of skills in shipyards and the supply chain.”
Sending the deal overseas would trigger uproar.
Since Brexit, the Government has not had to put such deals out to foreign competition.
Designating the vessels as warships, as the Defence Secretary did in September 2020, also meant the deal did not have to go to international tender - but the Tories chose to anyway.
As well as Spanish state-owned Navantia, Team Resolute includes Belfast-based Harland & Wolff, which made the Titanic, and London-based designers BMT.
Other bids shortlisted include one from Mumbai-based Larsen & Toubro, which describes itself as “India's premier infrastructure developer”, and another from Dutch firm Damen Group.
The four bids were each given £5million by the Ministry of Defence to develop their offerings as the Government began the “competitive procurement phase”.
GMB national officer Matt Roberts said tonight: “At a time of international uncertainty and with the UK economy in crisis, it seems utterly perverse for the Conservatives to send a massive naval contract overseas.
“Sending parts of the FSS order to Spain rather than delivering all the work in the UK is an insult to shipyards across the UK.
“Once again the Conservatives are showing they cannot be trusted with our nation’s security.”
Labour has pledged to build the ships in Britain.
An MoD spokeswoman said: “The contract for the Fleet Solid Support competition will be awarded to a UK business, either solely or as part of a consortium, promoting growth across the UK shipbuilding sector.
“As the competition is ongoing it would not be appropriate to comment further at this time.”
