Jack McMillan won the men's 400m freestyle at the European Short Course Championships in Poland, as Daniel Wiffen finished third.
McMillan, an Olympic and World champion in the men's 4x200m relay, claimed the gold medal ahead of Germany's Lukas Martens.
AdvertisementAdvertisementAdvertisementWiffen took bronze for Ireland in his first appearance since he had surgery to remove his appendix in August.
The Olympic and world champion set the fastest time in the heats with a time of 3:37.04, but had to settle for third in the final.
In a thrilling race, McMillan beat Martens to gold by just 0.18 seconds, and Wiffen was just 0.69 seconds off his former team-mate.
McMillan, from Northern Ireland, used to swim for Ireland but changed to represent Great Britain in 2022.
After helping Team GB to Olympic gold in the men's 4x200m in Paris, he repeated that feat in the World Championships in Singapore in July.
AdvertisementAdvertisementAdvertisementIn a tight final, Great Britain's Duncan Scott finished in seventh place.
Wiffen will return to the pool on Wednesday for the 1500m heats and will compete in the 800m on Friday.
McMillan will look to add to his 400m gold when he races in the 200m heats on Wednesday and the 100m heats on Friday.