Rebecca Adlington today broke the Olympic record to stay on track for double gold in Beijing as Mark Foster's fifth Games ended in less than 23 seconds.

The 19-year-old, who won the 400metres freestyle on Monday, smashed the previous Olympic best by over one-and-a-half seconds in her heat for the 800m freestyle.

At the halfway point, she was even on course to take Janet Evans's world record - which was set when Adlington was just six months old - before finishing in 8 mins 18.06 secs, less than two seconds outside one of the most illustrious marks in swimming.

The previous Games best was 8 mins 19.67 secs set by America's Brooke Bennett in Sydney eight years ago.

Adlington will be joined in the final by compatriot Cassie Patten, who set a personal best to qualify as the eighth fastest swimmer.

"I can't believe it," said Adlington. "I didn't expect to do that time at all.

"My aim was to improve on what I did at the trials and I did that so I'm happy. It was going to be a fast final so I knew I couldn't afford to ease back."

Foster cut a dejected figure after a poor start in his heat meant he finished outside the top-16 qualifiers for the 50m freestyle semi-final.

The five-time Olympian, who carried the Union Jack during the opening ceremony, said: "In the warm up I felt rubbish, I did a couple of sprints and felt amazing so I thought it was going to be brilliant - then I dived in and felt awful again.

"I don't have any excuses. It was the same time as I did in qualification in Sheffield. Going faster in June and doing a time that would have got me in the semis is gutting and I'm annoyed that I'm not going back out there because it's what I enjoy."

Foster came out of retirement in a bid to qualify for the Olympics after missing out on selection for the team for Athens four years ago but, after his failure to progress in Beijing, he refused to confirm if he was calling time on his career again.

The 38-year-old said: "I'm going to keep myself in shape. I'm not saying if I'm going for another year, four years or whatever."

Lizzie Simmonds, meanwhile, set a British record in the 200m backstroke as she progressed as second-fastest qualifier for the semi-finals. Team-mate Gemma Spofforth also went through.

Earlier in the day, Britain's Fran Halsall reached the final of the 100m freestyle, while Liam Tancock and James Goddard qualified for the men's 200m individual medley final.

Elsewhere, Britain's Gregor Tait made it through to his second straight Olympic 200m backstroke final.