The heavy-lift rocket, which is already years behind schedule, was just 30 minutes away from launching on Monday when a faulty valve meant the mission was postponed for a third time.

The mission carrying a French military satellite comes as Europe seeks to strengthen its defences amid doubts it can still rely on the United States as a security partner under new US President Donald Trump.

A successful launch will also mean that Europe can independently put large satellites into orbit for the first time since Russia pulled its heavy Soyuz rockets after invading Ukraine in 2022.

The launch is now scheduled to blast off from Europe's spaceport in Kourou, French Guiana at 1:24 pm (1624 GMT) on Thursday.

Monday's launch was called off at the last minute because of a dysfunctional valve on one of the refuelling pipes, according to the French company Arianespace which operates the rocket.

Tests before the launch showed "abnormal behaviour," so the only choice was to postpone, Arianespace chief David Cavailloles explained.

The rocket and satellite are "stable and safe," Arianespace said.

Previous attempts to launch the mission in December and February were also scrubbed.

Postponed launches are common for new rockets. The latest test flight of the world's biggest rocket, SpaceX's Starship, is also planning a Thursday launch after a last-minute cancellation on Monday.

- 'Europe must ensure its own security' -

Ariane 6 was initially scheduled to have its first fight in 2020, but repeated delays meant the rocket did not blast off for the first time until July last year.

Its first commercial mission is to put the CSO-3 satellite into orbit at an altitude of around 800 kilometres (500 miles) above Earth.

CSO-3 has been waiting since 2022 to complete a network of three French military imaging satellites, with the first two launched in 2018 and 2020 on Soyuz rockets.

Europe has just 10 military satellites -- five French and five Italian -- compared to "hundreds" for the US and China, French General Philippe Steininger said.

"Europe must ensure its own security," the European Space Agency's space transportation director Toni Tolker-Nielsen said in Kourou earlier this week.

He called for the number of annual Ariane 6 launches to increase to 12, compared to the five planned for this year.

Given the military role of the satellite, strict security precautions were taken to limit access at the spaceport on the northern coast of South America, with French fighter jets deployed to patrol the surrounding skies.

The European space industry has struggled to remain competitive with Elon Musk's SpaceX, which has become increasingly dominant when it comes to launching satellites.

And SpaceX is only expected to become more central to US space efforts with the billionaire Musk becoming a prominent advisor to Trump.

Europe recently found itself without a way to independently launch missions into space due to the Ariane 6 delays, its predecessor Ariane 5 retiring in 2023, Russia pulling its Soyuz rockets and an accident that grounded the smaller Vega-C launcher for two years.

But between Vega-C resuming flights in December and Ariane 6's first commercial launch, European space efforts are hoping to turn a page on the crisis.

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