The Cable and Wireless team, which recently passed over the western Sahara desert, have had to delay the operation until first light on Saturday because of an "inversion layer" of air pushing the balloon too high.

Pilots Andy Elson and Colin Prescott will have to lower the balloon to 9,000ft (2,700m) before pilot Mr Elson can open the roof hatch on their pressurised capsule and climb outside.

He will be wearing a harness and parachute for the walk during which he will abseil twice down the side of the capsule to examine the fuel tanks.

He will also examine one of the seven cameras fixed to the outside of the gondola which provide the pair with views of crucial equipment.

Organisers say the repairs are vital to the future of the record attempt and that Mr Elson is the right man for the job.

Jet stream hopes

Prescot and Elson (right) before take-off

The balloon is currently travelling at an altitude of just over 18,000ft (5,400m) approaching the border of Mauritania in north west Africa.

Mr Elson has performed a sky walk before during his unsuccessful bid to circumnavigate the globe in 1998, but has said he hoped he would not have to repeat the experience, which left him feeling "exposed."

The balloon has travelled only some 800 miles (1,300km) since take-off on Wednesday but the pilots are hoping to pick up the east-bound tropical jet stream to take them across Africa and the Himalayas at an altitude of 40,000 or 50,000 feet (15,000m).

"At that time their speed will increase dramatically," Flight Director Ian Ashpole said from the mission headquarters in London.

Permission denied

But a BBC weather forecaster said the jet stream still lies some distance to the south of the balloon's current location and she feared the wind would carry the balloon north east over China, which has not given overflight rights.

Permission has been given by Libya and Iraq both of which have denied it to previous round-the-world attempts.

Mr Ashpole described the balloon's launch from a golf course launch pad near Almeria in southern Spain on Wednesday morning as "superb".

The only problem was a leaking top hatch, which Mr Ashpole said had been resealed.

If all goes well, a successful touchdown could come in the North Sea or in the Mediterranean anywhere from 12 to 25 days after lift-off.

Circumnavigating the globe non-stop in a balloon is one feat yet to be accomplished. The Anheuser-Busch company is offering a prize of $1m to the first team that makes it.