Roberto Pavoni and Joe Roebuck failed to make it through to the final of the 400 metres individual medley which saw defending champion Michael Phelps only just scrape through on the first morning of Olympic competition.
Pavoni, who grew up just down the road in Harold Wood, will have been disappointed not to have made it given he reached the final of the World Championships last year in Shanghai.
Instead he finished 13th with Roebuck 24th although there was almost a huge shock as Phelps only just made it through in eighth while Beijing silver medallist Laszlo Cseh was edged out.
Fastest through was Japanese swimmer Kosuke Hagino while world champion Ryan Lochte recorded the third best time although he was clearly easing down when caught by South Africa's Chad Le Clos.
Roebuck was the first home swimmer in the pool and his appearance behind the blocks was greeted by a huge cheer at the Aquatics Centre.
The Rotherham-born swimmer led after the opening butterfly leg but he fell behind on the backstroke, to turn seventh at the halfway point.
The Loughborough ITC athlete then found himself in eighth with 100m to go and could not make any inroads to touch in four minutes 20.24 seconds although a disqualification bumped him up.
Pavoni was alongside Beijing triple silver medallist Cseh and one removed from Phelps.
The 21-year-old produced a strong breaststroke leg to find himself in fifth going into the final freestyle 100m.
Fifth at the last turn, he clawed away and managed to touch in fourth in 4:15.56.
Phelps had vowed never to contest the event again after Beijing but after qualifying at the US trials last month, the Baltimore swimmer decided to give it one last go.
His appearance prompted an ovation as he commenced his final Olympic programme, retirement beckoning after London.
First after a commanding butterfly, it was Hungarian Cseh who entered the final freestyle leg first, but the American clawed it back to touch in 4:13.33, ahead by 0.07.
Cseh's failure to progress was a shock and Phelps will breathe a sigh of relief that he edged through.
While Phelps has already secured 14-time Olympic titles, it is Lochte whose star has risen over the last couple of years, culminating in five golds in Shanghai last summer.
The Florida-based swimmer won last month's US trials and he looked fearsome this morning. Despite being caught by Le Clos at the touch, the American was clearly easing down to touch in 4:12.35.
Roebuck said he found out last night he was the first Briton when he saw the start sheet.
"I didn't think about it really until this morning and thought it was kind of cool."
Of the atmosphere, Roebuck added: "I think it'll only get better throughout the week.
"It did feel fantastic to walk out there and be the first guy for British swimming this weekend.
"Just a bit disappointed for everybody else that I didn't kick it off with a swim that everybody would have wanted."
Phelps said: "I didn't expect those guys to go that fast.
"I just wanted to try to get some good underwater, try to get some good times.
"A final spot is a final spot.
"I was slower this morning than I was four years ago, so I guess that's probably the only difference.
"I think the only thing that matters is really getting a spot in.
"You can't win the gold medal from the morning."
Lochte said it had not been his best race.
"I'm glad I got the cobwebs out. All I had to do was get a lane for tonight, so I'm very happy.
On his friend and rival Phelps, Lochte said: "It's a tough field but he's in, so you can't count him out, even though he just squeaked in in eighth place.
"He's a racer and we're going to do everything we can to go one-two tonight."
Fran Halsall was the first British woman to make it into the pool in heat four of the 100m butterfly.
The Loughborough ITC swimmer's favourite event is the 100m freestyle which comes later in the week.
Today she was second in her heat in 58.23 and eighth overall going into tonight's semi-finals.
Ellen Gandy, fancied for a medal in the 200m butterfly after world silver last year, was also second in the following heat in 58.25, going through in ninth.
However, American Dana Vollmer made a statement of intent, the 2011 world champion setting a new Olympic record of 56.25, 0.19 outside Sarah Sjostrom's 2009 world record set during the shiny suit debacle.
Halsall said: "It's exciting actually because when my name came up on the board just before we were about to start, all I could hear was the crowd going, 'Whey!', and they're blowing the whistle for everyone to be quiet and everyone's still shouting, so it really made me smile and relax.
"The first one's always a bit more nervous than the rest because you want to know how you're going and how you're feeling and just blow the cobwebs away.
"So it's nice to have that extra bit of relaxation because I was giggling about the crowd shouting. So, yeah, I was very happy with that."
The 22-year-old admitted the butterfly was a warm-up for the free, saying: "Yeah, definitely. Just to sharpen up a bit and get a bit of race practise before my main one.
"So I just want to sharpen up, make sure everything's where it needs to be, soak up the atmosphere and just enjoy it - and I could do that with the fly.
"So it's perfect."
Halsall described the atmosphere as: "Amazing. Absolutely fantastic. I loved it."
Vollmer looked fearsome and the American said: "It felt great. I'm always a little more nervous for my first one and I just wanted to see where I would go for speed and make sure that it's the place I wanted to go so that, when I'm under pressure, that's where it happens from.
"I'm really happy with how fast it was and I feel like it's only going to get faster."
As for the possibility of her breaking Sjostrom's world record, she said: "Hopefully."
There were further shocks in the men's 400m freestyle when defending champion Park Tae Hwan was disqualified for an apparent false start.
Neither did world record holder Paul Biedermann make it through.
However, David Carry became the first Briton to book a final spot when he qualified seventh in 3:47.25.
Robbie Renwick, though, failed to progress in 10th (3:47.44).
Carry, the oldest member of the team at 30, said: "It's a dream come true, really, to be in a final of the Olympic Games.
"It was my goal this time last year to do that.
"Not many people believed I could do it but I turned it round and just absolutely thrilled."
Renwick added: "It was a tough swim but I'm quite pleased with the time.
"I would've liked to have gone faster."