The project, begun in 1989, was originally expected to deploy the instrument -- which will be the largest and most powerful telescope ever to be launched into space -- in the early 2000s.

But multiple problems forced delays and a tripling of the telescope's original budget with a final price tag of nearly $10 billion (8.8 billion euros).

The Webb telescope was built in the US and transported to its launch site in Kourou in French Guyana this year with a planned date of departure of December 18.

However, new problems have forced two delays.

"The James Webb Space Telescope is confirmed for the target launch date of December 24," tweeted launch company Arianespace, adding that it would go ahead at 12:20 GMT on that day.

Confirming the launch date, NASA tweeted that the telescope was "encapsulated inside its @Ariane5 rocket fairing".

It follows in the footsteps of the legendary Hubble telescope but will be located much further from the sun. It is hoped it will reveal what the Universe looked like even closer to its birth nearly 14 billion years ago.

The new telescope is named after the late James E. Webb, who ran the fledgling NASA space agency for much of the 1960s.