* Britain win world sprint relay for first time
* Bolt pulls up injured running last leg of final race
* United States take silver, Japan bronze (Adds quotes, detail)
By Mitch Phillips
LONDON, Aug 12 (Reuters) - Usain Bolt’s final track appearance ended with pain and frustration on Saturday as he pulled up injured running the final leg of the World Championships 4x100 metres relay as Britain stunned the United States to win a shock gold medal.
Bolt, who had to settle for bronze in the individual 100 metres, had been hoping to sign off from the sport by leading Jamaica to a fifth successive world relay title but they were struggling in a distant third place when he collected the baton.
As he tried to gain ground, Bolt pulled up and fell to the floor midway down the home straight with what was later described as cramp in his left hamstring.
However, the 60,000 in the stadium who had come to mark the farewell of sport’s greatest showman had only a split second to absorb what was happening as up ahead history was being made by the host nation.
The United States, with individual 100m gold and silver medallists Justin Gatlin and Christian Coleman running the second and fourth legs, had been expected to push the Jamaicans all the way but they were always trailing the slick Britons.
Brilliantly executed exchanges by Chijindu Ujah, Adam Gemili, Danny Talbot and Nethaneel Mitchell-Blake gave Britain gold in 37.47 seconds, breaking an 18-year-old national record and European record and securing the world title for the first time.
“I wasn’t sure if I had won or not, I gave it my all but I could see Christian Coleman out of the corner of my eye,” said Mitchell-Blake.
“The feeling of euphoria was from infinity. I can’t register it. We smashed the British record to pieces.”
Five years ago Talbot and the 18-year-old Gemili messed up a changeover as Britain were disqualified in the semi-finals of their own Olympics on the same London track so it was a particularly sweet moment for the two.
“To run it with Danny Talbot after such disappointment in London 2012, it is so special to come back,” said Gemili, who was not selected for the individual races and missed an Olympic 200m bronze by three thousandths of a second last year.
“It’s crazy. Honestly a dream and a reality tonight.”
Britain’s only other men’s global sprint relay golds came in the 1912 and 2004 Olympics while their winning time that beat their 1999 best of 37.73 has only been bettered by Jamaica and the U.S.
It completed a great night for the host nation’s sprinters after their women had earlier taken silver behind the Americans in the sprint relay, their best performance since the first championships in 1983.
The U.S. men took silver in 37.52 but for a nation that used to be totally dominant in the event, it was only their second medal in the last eight global finals after a series of world and Olympic disqualifications.
“After DQs in recent years we knew we had to get the baton round and to finish the night smoothly is a fantastic achievement and we’ll enjoy our moment on the podium,” said Gatlin.
“The Brits had a hell of a race. You can’t take it from them. They had the pressure of running on home soil and they delivered.”
On his old rival Bolt he said: “This is farewell time, I am sentimental about it already now.”
Japan took advantage of Jamaica’s travails to take third in 38.04, just holding off China (38.34). (Editing by Ed Osmond)