Jeremy Corbyn has called for shipbuilding contracts linked with the Navy to stay in the UK in a speech in Glasgow.
On a visit to the historic shipbuilding heartland of Govan on Friday, the Labour leader said three new Royal Fleet Auxiliary ships must be built in Britain.
The UK government plans to put the £1 billion contract for the ships out to international tender later this month and has faced union calls to reverse the decision.
Mr Corbyn accused the Conservatives of "trashing" the UK's shipbuilding tradition and attempting to accelerate its decline.
He said: "Workers in British shipyards, from Plymouth to Rosyth, share a proud tradition - building the best ships in the world.
"But the Conservative government is trashing that tradition by offering up the Ministry of Defence's most recent contract for three new Fleet Solid Support Ships to overseas companies to build abroad.
"This decision is wrong. Today we are calling on the Government to guarantee that these three new ships for the Royal Fleet Auxiliary will be built in domestic shipyards."
Mr Corbyn said building the ships in the UK could secure more than 6,500 jobs - 1,800 of these in shipyards.
"Our shipyards used to produce half of all new ships worldwide. Our current market share is now less than half a percent. The Tories seem hell-bent on accelerating and deepening this industrial decline," he added.
"The next Labour government will use public contracts as part of our bigger plans to upgrade our economy. Don't listen to anyone who says we can't build things in Britain and that a casino economy, which produces little but soaring inequality and insecurity, is our only future.
"Shipbuilding is not a lame duck, and can have a high tech, high skilled and exciting future right here in the UK."
"The Government claims that it is overseeing a 'renaissance' in British shipbuilding. But Scottish employers have pushed through one set of redundancies after another in recent years.
"Our proposal would both sustain existing shipbuilding and supply chain jobs, and create new ones - right here in Scotland and also across the UK."
An MoD spokesman reiterated that Britain was witnessing a "renaissance in national shipbuilding", saying all its warships are built in the UK, with the Clyde securing 4,000 jobs until 2035 having been given a commitment it will build the Navy's eight planned Type 26 Frigates.
He added: "We are launching a competition for three new Fleet Solid Support ships this year and strongly encourage British yards to take part.
"Since 2010 this Government has invested more than £6 billion in shipbuilding in the UK, securing thousands of jobs. While in 2018-19 we expect to spend in excess of £750 million supporting the fleet."
Chuka Umunna MP, for the People's Vote campaign calling for a vote on the final Brexit deal, said:
"Labour is already committed to remaining in a Customs Union with the EU and combining that with a pledge to negotiate to stay in the EEA would demonstrate a deep commitment to the well-paid and highly skilled jobs the engineering sector supports in the west of Scotland and elsewhere."
SNP MSP for Glasgow Anniesland, Bill Kidd, said:
"Jeremy Corbyn has some brass neck coming to Glasgow to make more empty statements on shipbuilding.
"Workers on the Clyde and people across Scotland haven't forgotten Labour's betrayal of the industry in 2014 - making promises they couldn't keep in order to shore up votes in their grubby alliance with the Tories.
"Scotland was promised 13 new frigates, then it was eight, but they've only signed contracts for three. Despite the Tories' track record of failing Scotland, Labour happily trusted them anyway. It is shameful."