Briton Tim Peake's spacewalk has ended after a water bubble was reported in the helmet of his fellow astronaut.

NASA tweeted that the mission had been terminated due to a "small amount of water" in Tim Kopra's helmet.

The two astronauts had already successfully restored full power to the International Space Station (ISS) by replacing a broken electronic box.

But after Kopra reported the bubble, Mission Control decided to cut the six-hour spacewalk short - ending it after four hours and 10 minutes.

Both astronauts were safe, according to the ISS.

"So far, I'm ok," Kopra assured everyone.

Later, he said the water bubble was four inches long and getting thicker.

Today’s exhilarating #spacewalk will be etched in my memory forever – quite an incredible feeling! pic.twitter.com/84Dn3gHA70

— Tim Peake (@astro_timpeake) January 15, 2016

Live footage showed both astronauts safely returning to the airlock area of the ISS and the thermal cover being closed behind them.

Once in the main body of the station, their colleagues inside will help with a 25-minute clean-up and further checks.

NASA stressed that the situation was not an emergency.

Tweeting from its official account, NASA said: "As a precaution, spacewalk terminated due to small amount of water in Tim's helmet."

Later, the ISS tweeted: "@Astro_Tim & @Astro_TimPeake thank ground support after wrapping up #spacewalk and completing major installation."

Major Peake also posted a selfie from the spacewalk and said: "Today's exhilarating spacewalk will be etched in my memory forever - quite an incredible feeling!"

Earlier the astronaut, from Chichester, West Sussex, told of his pride in stepping into space with the Union flag on his space suit.

He is on a six-month mission with the European Space Agency (ESA).

As he prepared to exit the air lock at the start of spacewalk, one of Major Peake's colleagues, Commander Scott Kelly, remarked it was "really cool" to see the Union flag outside.

Mr Kelly said: "The Union Jack has explored all over the world, now it's exploring space."

Major Peake replied: "It's great to be wearing it. A privilege, a proud moment."

His wife, Rebecca Peake, left a short message for her husband as he left the ISS.

"@astro--timpeake thank you for taking our boys with you into the vacuum of space," she wrote on Twitter.

Back on earth, well-wishers also took to Twitter to express amazement and wish the astronauts good luck.

Sir Paul McCartney wrote: "We're all watching, no pressure! Wishing you a happy stroll outdoors in the universe."

During the spacewalk, the two men remained tethered to Space Station supports.

Their suits had built-in jet packs in case they accidentally drifted away.