Three Scots - Stephen Milne, Duncan Scott and Dan Wallace - set up world champion James Guy to bring home the GB quartet for a superb silver in the 4x200m freestyle relay, while Siobhan-Marie O’Connor took the same colour of medal in the women’s 200m individual medley.
Another Scot, veteran Robbie Renwick, will also get a medal as he was part of the British relay team in the heats.
Phelps anchored home the Americans to win his second gold of the evening, his 25th Olympic medal in total.
Britain are world champions and qualified fastest for the final, before drafting in Guy in place of Renwick.
Guy combined with the Scottish trio to clock 7:03.13 as the USA won in 7:00.66. Japan were third in 7:03.50. Australia were fourth.
Milne, the 22-year-old from Perth, was on the first leg and touched the wall in fifth place, but Scott, on the second leg, moved Britain up to fourth.
Wallace, the Commonwealth 400m individual medley champion who was initially selected as a reserve, pushed Britain into third place with a storming third leg, leaving it up to Guy, the world 200m champion who had struggled to fourth in the individual event on Monday.
Phelps was so far ahead it was something of a procession, with Guy battling Japan, Australia and Russia for the podium places.
But the 20-year-old from Bury delivered, touching ahead of his rivals as Phelps and co celebrated.
It capped a brilliant day for 19-year-old University of Stirling student Scott, who broke the British record in yesterday afternoon’s 100m freestyle heat, clocking 48.01 to advance third fastest.
Against some of the big beasts of male swimming, the Scots teen then clocked 48.20 to finish fourth in his semi-final and qualify for tonight’s final (Thu 3.03am) in seventh place.
Siobhan-Marie O’Connor won 200 metres individual medley silver as Great Britain’s swim team equalled the number of medals claimed in swimming at London 2012 before the relay silver surpassed that total.
The 20-year-old Bath swimmer was the fastest qualifier and 400m individual medley champion Katinka Hosszu of Hungary showed how much of a threat she considered the young Brit when she withdrew from the 200m butterfly earlier in the evening to concentrate on the IM final.
Hosszu was under world record pace for much of the race and finished in two minutes 06.58 seconds, an Olympic record.
O’Connor smashed her own British record to take silver in 2mins 06.88secs, while Maya Dirado of the United States was third in 2:08.79. Defending champion Ye Shiwen of China was eighth in 2:13.56.
Britain’s swimmers attracted criticism for winning just one silver and two bronze medals four years ago in home waters but now have four medals, including Adam Peaty’s gold, in the bag already.
Phelps won his 20th Olympic gold with a dominant victory in the 200 metres butterfly.
South African Chad Le Clos shocked Phelps by winning gold ahead of his idol, in Phelps’ signature event, at the London 2012 Games.
Phelps had won the title at the 2004 and 2008 Olympics and reclaimed it with a block to wall victory in one minute 53.36 seconds.
Masato Sakai of Japan threatened to come from behind to upset the American, finishing four one-hundredths of a second behind in second.
Hungary’s Tamas Kenderesi took bronze in 1:53.62 as Le Clos finished outside the medals in fourth in 1:54.06.
An unpredictable Olympic tennis tournament claimed its biggest scalp of all as Serena Williams crashed out in the third round of the women’s singles.
The world No 1 appeared to have a shoulder problem and was close to tears as she lost 6-4, 6-3 to Ukraine’s Elina Svitolina.
Serena and sister Venus are the most successful players in Olympic tennis history with four gold medals each but Rio has been dreadful for the family.
Venus lost in the opening round of the singles to Kirsten Flipkens while the sisters lost their first ever Olympic doubles match on Sunday.
Serena won gold in both events in London four years ago and appeared to have put doubts about her form to bed by winning her 22nd grand slam singles title at Wimbledon last month.
The 34-year-old’s exit means Angelique Kerber and Madison Keys are the only top-eight seeds remaining of what had appeared a very strong field.
Earlier, Britain’s Andy Murray and Johanna Konta both came through their second round matches. Murray beat Juan Monaco of Argentina 6-3, 6-1 and will next face Fabio Fognini of Italy, while Konta won an epic tussle with Svetlana Kuznetsova of Russia 3-6, 7-5, 7-5 and next faces Kerber.
The majestic Simone Biles landed the first of a possible five Olympic gymnastic golds in Rio by helping the United States win the women’s team title.
The 19-year-old led the dominant Americans to an expected victory as Great Britain’s women had to settle for fifth place, joining their male counterparts in missing out on a medal.
Biles produced the best vault, beam and floor routines of the competition as the US led from start to finish, finally winning by a huge margin of more than eight points.
The team, also comprising Lauren Hernandez, Alexandra Raisman, Gabrielle Douglas and Madison Kocian, took gold with a score of 184.897, with Russia taking silver and China bronze.
Britain took bronze at last year’s World Championships in Glasgow and there was little separating them from the medals here, but mistakes cost the quintet of Ruby Harold, sisters Elissa and Rebecca Downie, Amy Tinkler and Claudia Fragapane dear.
They missed a medal by just over one-and-a-half points, finishing with a score of 174.362.
Green not gold for dive duo
Tonia Couch and Lois Toulson joked they would be asking for a green pool in future after recording a personal best in finishing fifth in the women’s synchronised 10metres platform final at the Rio Olympics.
The pool appeared to turn greener as the final wore on and reached a near-inevitable conclusion: victory for China, their third from three events at the Maria Lenk Aquatics Centre.
“It’s so green,” Couch said. “But we got a personal best score, so maybe we should ask for a green pool from now on. I kind of liked it.”
Couch and 16-year-old Toulson laughed off the darkening deep.
“I couldn’t see you when I was underneath. I was like ‘Lois?’” added Couch, motioning as if she was peering through the gloom.
The pair had no concerns about the water quality, despite the bottom of the pool becoming invisible as the final wore on and the contrast with the adjacent water polo pool became more striking.
The Rio 2016 organising committee is investigating the cause of the colour change, but insists there is no reason to be alarmed.
“Water tests at Maria Lenk Aquatics Centre diving pool have been conducted and there was found to be no risk whatsoever to athletes’ health,” a venue spokesman said.
“We’re investigating what the cause of the situation was. I’m happy to report the competition was successfully completed.”
There were some suggestions from observers with pool maintenance experience that the chemical balance in the water was incorrect.
Twenty-four hours earlier the water was clear, when Tom Daley and Dan Goodfellow won bronze in the corresponding men’s event.
It was not to be for Couch and Toulson, who finished with a score of 319.44 points as Chen Ruolin and Liu Huixia of China won with 354.00pts.
Silver went to Cheong Jun-hoong and Pamg Pandelela Rinong of Malaysia with 344.34 and Canada’s Meaghan Benfeito and Roseline Filion took bronze on the final dive, finishing with 336.18 to displace North Korea from the podium.