Natalie Massey broke the 400m freestyle world record on the last day of the British International Disability Swimming Championships in Sheffield.
The Manchester-based swimmer competes in the S14 category for athletes with learning disabilities and lowered the European record in the morning heats.
She paced herself well through the first 200m and finished strongly in four minutes 44.48 seconds.
"I'm really pleased but shocked because I'm in heavy training," she said.
"Once I'd got the European record in the morning, I knew I had the world record in me too.
"Doing the 400m free really helps with my 200m because I can pace it and see what I'm doing. This has really given me confidence for the European Championships now."
Her time eclipsed the previous British best of 4:48.97 seconds.
Double Paralympic champion Eleanor Simmonds put in a strong performance to swim close to her S6 world record and take silver behind Massey.
Massey's club-mate Jonathan Fox won the men's 400m freestyle after swimming closest to the world record in his S7 category, ahead of Sam Hynd (S8) and Robert Welbourn (S10)
Natalie Jones and Susie Rodgers both lowered the British records in the 50m butterfly, while Ben Procter (S14) claimed his second gold medal of the competition and also secured the qualification time for the European Championships in the 200m freestyle.