Astronaut Tim Peake has described the moment he became the first Briton to walk in space as a "real privilege".
The 43-year-old, from Chichester, West Sussex, ventured out of the International Space Station on Friday to help repair a broken power unit.
A live feed on the Nasa website showed the moment Major Peake tentatively emerged from the ISS with the Union flag on the shoulder of his space suit.
As he prepared to exit the air lock, one of his colleagues, Commander Scott Kelly, told Major Peake 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."
Before the pioneering moment he was pictured with his Nasa colleague Colonel Tim Kopra arranging his tools and getting into the airlock.
Prime Minister David Cameron said on Twitter hours before the spacewalk: "Good luck to @astro-timpeake on today's #spacewalk."
He added: "The country will be watching you make history."
Major Peake said he was "exhilarated" by the thought of walking in space, which will take longer than six hours.
On his blog he wrote: "Although I am exhilarated by tomorrow’s spacewalk I have no time to dwell on these emotions.
"The six hours and thirty minutes we will work on the Space Station’s hull are meticulously planned and Tim and I need to execute each step methodically.
"Spacewalks, like many critical operations, operate on the buddy-buddy system.
"Tim and I will constantly be checking each other and relying on each other for assistance if something should go wrong."
The European Space Agency (ESA) said every detail of the spacewalk was "choreographed minutely".
Major Peake began to breathe pure oxygen two hours beforehand, because the pressure inside their suits is lower than that of the ISS.
The pair then entered an airlock before opening its hatch and heading outside.
Like rock climbers, the astronauts must always be tethered to Space Station supports.
They successfully restored full power to the ISS after replacing a broken electronic box, working in darkness to avoid an electric shock from the solar power system.
However the space walk was terminated early after four hours and ten minutes for safety reasons, after Colonel Tim Kopra reported a small amount of water and dampness in his helmet's absorption pad.
Major Peake arrived at the ISS on December 15 and will stay for six months.
