British astronaut Tim Peake is set to become the first Briton to walk in space.

Today he will undertake his first space walk when he ventures out of the International Space Station (ISS) to help repair a broken power unit.

The spacewalk - which Nasa terms an Extra-Vehicular Activity (EVA) - will last almost six-and-a-half hours, and is scheduled to begin at 12.55 GMT.

In a tweet sent on Monday, Major Peake posted an image of himself in the spacesuit he will wear for the spacewalk, saying "feels just great".

He received words of encouragement ahead of the walk from fellow astronaut Chris Hadfield.

On Twitter, the Canadian astronaut said, "See the world for us, Tim."

On leaving the space station, Major Peake will be led his Nasa colleague Colonel Tim Kopra, 52.

The two have spent weeks preparing, working through a timeline of the spacewalk that is almost 40 pages long.

Colonel Kopra will go first, heading to the solar units that need to be repaired.

Once given the green light Major Peake will follow with the replacement equipment.

The pair should finish the repairs in under three hours and will move on to lay cables for new docking ports and reinstall a valve that was removed last year.

The six hours will be tiring for the pair as they fight against their pressurised suits and they face a number of difficulties.

As they move towards the edge of the Space Station they will be secured by safety tether - a thin steel wire on a reel - but they must be careful not to become tangled in it.

Sunrises can also be "blinding for moments" from space, according to the European Space Agency (ESA), and they will need to check on each other frequently and make sure their suits have no damage.

The ESA warned: "A spacewalk is a test of patience and alertness."