James Guy made history with gold in the 200m freestyle and Adam Peaty set a 50m breaststroke world record as British swimmers continued their charge at the world championships.

Great Britain’s most successful world championships in the pool came in 2003 in Barcelona when they won eight medals but they have already secured five – with only three days of the eight-day event gone.

Guy had won silver in the 400m freestyle on Sunday and was a medal contender going into the 200m final. The 19-year-old was third at the final turn behind China’s double Olympic champion Sun Yang – who was going for a clean sweep of all the freestyle events from the 200m upwards – and the American Ryan Lochte, the 23-time world medallist.

The Millfield swimmer overhauled the pair down the last 50m and held off Sun, who had beaten him in the 400m, to touch in a national record of 1min 45.14sec, becoming the first British man to win a world freestyle title.

It was also only the second world 200m medal for a British man following Brian Brinkley’s bronze in Cali, Colombia, 40 years ago.

Guy said: “I never thought I would win the final of the 200 at all. I never thought I would race Ryan Lochte head-to-head like that, he is one of my heroes, so to race him head-to-head was amazing. To put my hand on the wall first and be world champion, it hasn’t sunk in yet but I am so happy.

“I just tried to focus on myself. I knew the boys would go out hard, so just tried to stay with them and not let them get too far away. Sun is a hero of mine: now I am with him. I’ve got a gold medal now. Obviously, he is a bit faster than me but hopefully it will come on next year. The Lochte underwater phases are like a dolphin: if I had them I could go quicker.”

Minutes later, Peaty had set a 50m breaststroke world record of 26.42sec in the first semi-final. The 20-year-old thought he had set a world mark of 26.62 at the European Championships in Berlin last year. An administrative error by governing body Fina, however, means it has never been ratified, so when Cameron van der Burgh matched that time in the heats, it was given as a world record.

Not that it matters now, with the Uttoxeter swimmer having taken the matter out of Fina’s hands.

It means he and Van der Burgh – the world champion – will renew acquaintance in the final on Wednesday, two days after Peaty won the 100m on the final stroke, the pair slugging it out like prizefighters over the course of the championships.

Peaty said: “It was a really good race, the heats this morning were so easy; 26.68 was probably the easiest 50 I have ever done. I knew there was something in there but I decided to just stay relaxed, enjoy the environment. The 50m event is not an Olympic event, so there is kind of no pressure. I just enjoyed what I was doing. Semi-finals are a good time to do it, you don’t have to worry about the medal places. It was great just to get out and put a good time on the board.”

Was he spurred on by seeing Van der Burgh set a world record in the morning when it, in fact, equalled his 2014 time? “Kind of. Unofficial world record. I am not going to get into the politics about it but he is still a great 50m swimmer, so hopefully the final is going to be a really good race. All I’ve got to do is get my start right.”

Chris Walker-Hebborn and Liam Tancock were fifth and eighth respectively in the 100m backstroke final, while Lauren Quigley finished seventh in the women’s equivalent.