EU officials say Brussels has asked the UK to pay up to 6.5 billion euros to secure more lucrative access to the programme for British arms manufacturers.

But London -- which left the EU in 2020 after years of bitter negotiations -- appears to have baulked at that initial price tag and is pushing for a better deal, diplomatic sources said.

"Nothing has been agreed, and we will not give a running commentary on talks," a British government spokesman said.

EU chief Ursula von der Leyen later said she discussed UK participation in the scheme in an evening phone call with British Prime Minister Keir Starmer.

"We continue the work together to achieve a mutually beneficial outcome," she wrote on social media platform X.

European nations are scrambling to bolster their militaries in the face of Russia's aggression almost four years into the war in Ukraine.

The SAFE scheme launched by Brussels earlier this year will provide EU countries with 150 billion euros of loans backed by the bloc's central budget to help them rearm.

EU countries are currently obliged to spend the money on weapons that are at least 65 percent produced in the bloc.

If Britain joins the scheme, that figure would be reduced in the case of the UK -- meaning British firms could try to cash in more.

"Such an agreement will indeed bring major economic and commercial opportunities for the UK," a European Commission spokesman said.

The spokesman said the EU executive had told London the entry fee could be reviewed later on if the programme did not prove as lucrative as expected for British firms.

"The commission remains fully engaged in the negotiations," he said.