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Britain's Helen Jenkins missed out on an Olympic triathlon medal, finishing fifth in Hyde Park as Switzerland's Nicola Spirig took gold.

Jenkins, the world champion from Wales, kept pace with the leaders with the help of team-mate Lucy Hall and looked set for a medal.

But a quartet accelerated away on the final lap, leaving her to finish fifth.

Spirig beat Sweden's Lisa Norden in a stunning, unprecedented photo finish, with Australia's Erin Densham third.

Jenkins, 28, had entered the Olympics in excellent race form, winning in San Diego, her last World Series race leading up to the Games.

But she told 91�ȱ� Sport a knee injury had since limited her training and had a critical effect on her ability to finish off the run.

"After San Diego, I had a problem with my knee and since then it's been a battle to get here really. It hasn't gone away," she said.

"I've been training through a lot of pain for 10 weeks. We've had a lot of tears, a lot of emotion going on, but I've had a great team around me and we were confident I'd still be close to it.

"Unfortunately, I wasn't 100% there on the run. I could focus on my swim and bike, get that ready, but I didn't have enough today. If the race had been a few months ago, I would've been all right."

Hall, the youngest triathlete in the race at the age of 20, had been entered specifically to help Jenkins towards a medal and can be satisfied with her day's work.

Having led the 1500m swim, she stalled a breakaway of seven cyclists and then - once Jenkins had caught up - piloted the medal hope through the 43km without difficulty, chasing down a few abortive attempts by rivals to establish a gap on the bike.

Hall burned up on the 10km run, as expected, to finish 33rd. Vicky Holland, the other Briton, was caught in a crash that ended Australian hopeful Emma Moffatt's race early in the bike and came home 26th.

"It was on the first lap around Buckingham Palace. I think it was one of the white lines that was still a bit wet from the rain this morning," said Holland.

"I saw Emma go down, then I think someone took out my bike wheel. From there on in, it was a case of cruising around at the back and waiting for the run."

Jenkins initially looked well-placed on the run alongside Spirig, Densham and Norden, roared on by magnificent crowds up to 20-deep under sporadic sunshine.

But fifth place means Britain remains without a medal in the four Olympic Games since triathlon was introduced to the programme.

Bridgend's Jenkins entered the 2008 Beijing Games as world champion, as she did here, but could only finish 21st.

Britain's medal record may well change on Tuesday, when the men's race takes place over the same Hyde Park course.

Alistair Brownlee and younger brother Jonny are favourites, having dominated the men's sport for the past three years.