World championship silver medallist Ellen Gandy secured her second Team GB Olympic berth with victory in the 200m butterfly final at the British Swimming Championships.
She was two seconds under European record pace at the halfway point, but tired in the latter stages and only narrowly held off Welsh swimmer Jemma Lowe, who also qualified as runner-up.
"That's definitely the most painful race of my life," Gandy, who won the 100m butterfly title on Sunday, told BBC Sport.
"I'm not happy with how I swam. I was scared not to make it [qualify], but I'm so relieved that I got there."
The Australia-based 20-year-old added: "Coming back to London in the summer with all of the crowd and the support is going to be great."
Lowe, who finished just three tenths of a second behind Gandy's winning time of 2:06:01, missed out on an Olympic 100m butterfly place earlier in the competition.
"It's such a relief, it's been such an emotional week for me," said Lowe. "Before the race I thought I was going to pass out I was so nervous!"
Joining Gandy and Lowe at London 2012 will be Andrew Willis and Michael Jamieson.
The Bath-based 200m breaststrokers edged out British record-holder Kris Gilchrist into third.
"The last 50m was everything and I just went for it," gold medallist Willis told BBC Sport.
"Now I'll go back, have a break and then train hard and with it being a home Olympics we'll get lots of support and hopefully go even quicker."
Scotsman Jamieson, who missed out on an Olympic place in the 100m breastroke event earlier in the week, added: "I've had a nightmare of a week - none of the times or results I was looking for - but I'm so relieved right now."
The men's 100m freestyle is traditionally the sport's blue riband event, but not even winner Simon Burnett, who finished in a time of 49.33 seconds, was quick enough to qualify for the individual event at the London Games.
Burnett, James Disney-May, Craig Gibbons and Adam Brown should make the Team GB relay squad.
"I'm happy with that," said Burnett. "The qualifying time is pretty fast and for me to do it [at the second trials in June], it could compromise my Games, so I'm going to focus on the relay."
In the evening's multi-classification disability swimming finals, six-time European Champion Susie Rodgers underlined her potential for success later in the summer with an impressive victory and Paralympic qualifying time in the 50m butterfly event.
"It felt really good after five solid days of racing," said Rodgers.
"I came here to get the qualifying times and I've done that in most of my events, so I have to be pleased."
S12 swimmer Hannah Russell's 1:11:70 in the 100m butterfly put her closer to London 2012 qualification, which is likely to be confirmed at the second trials in Sheffield in April.
In the men's 50m butterfly final, six-time Paralympic champion Sascha Kindred took gold with Matthew Walker second, but neither achieved their desired qualification times.
It was a similar story in the men's 100m butterfly event, won by Rafael Bogott.
Amongst the highlights on day six of the championships will be the women's 100m freestyle final, when British record-holder Fran Halsall and Amy Smith renew their rivalry.
Commonwealth Champion James Goddard will contest the final of the men's 200m individual medley, although Ieuan Lloyd was the quickest qualifier.