The discovery of water in the helmet of a US astronaut brought an early end to a spacewalk at the International Space Station, NASA said.

"The crew is in not in any danger whatsoever," said NASA commentator Rob Navias after astronaut Colonel Tim Kopra reported feeling a glob of water about four hours into the spacewalk.

However, the situation brought back memories of an incident in 2013 when Italian astronaut Luca Parmitano's helmet began rapidly filling with water and risked drowning him.

"This is nowhere near as severe as that incident was," said Mr Navias.

British astronaut Tim Peake, who was embarking on his first ever spacewalk, checked his colleague's appearance while they were outside the International Space Station and described what he saw as a "film of water."

Col Kopra said the glob of liquid was about 10cm by 5cm long.

He also said it was cold water, indicating the leak may have come from a cooling loop inside the suit, said Mr Navias.

Col Kopra had reported a high carbon dioxide reading in his spacesuit earlier in the outing, but felt no symptoms, and mission control decided the alarm was due to a faulty sensor.

Shortly after flight director Royce Renfrew at mission control learned that Col Kopra was feeling dampness in his helmet, he decided to cut short the spacewalk.

By that time, the team's main mission - to replace a broken voltage regulator - had already been completed.

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.

As the airlock re-pressurised, the European Space Agency said the early termination of the spacewalk was officially completed at 5.31pm Irish time, four hours and 43 minutes in.

Col Kopra offered his thanks to the ground staff for ensuring their safety, a sentiment echoed by Major Peake.

He added: "Thank you very much, I just want to reiterate Tim's words there, you guys have done a great job, thanks for everybody's support."

His helmet-absorption pad, along with a sample of the water bubble taken with a syringe, will be analysed to determine what caused it to form.

Earlier today, Major Peake and Col Kopra switched their spacesuits on to battery power at 7.48am (12.48pm Irish time), marking the official start of the floating debut for the British astronaut.

As he climbed out of the space station, US astronaut Commander Scott Kelly positioned a camera from inside so that the flag on the arm of Major Peake's spacesuit was visible to viewers watching the outing live on NASA television.

"Great to see the Union flag out there," said Commander Kelly.

"It's great to be wearing it. It's a privilege," answered Major Peake.

Major Peake, a former Army Air Corps and helicopter test pilot, is the first Briton aboard the ISS and the first fully British professional astronaut employed by a space agency.

The International Space Station is expected to remain in operation until 2024.