“I could count the number of sprints I have done on my hand,” added Anderson. “I was more nervous for the pain of it.

“I did the job and am through to the final. Hopefully I can get the time down a bit tonight.”

Anderson won gold as part of Great Britain’s 4x100m mixed medley relay at the Tokyo Olympics. She also won bronze in 4x100m freestyle relay at the World Championships last year and was seventh in the 200m individual freestyle event.

Having revealed her diagnosis in March, she sat out the 200m freestyle at the Aquatics GB Championships this week before competing over the shorter distance – her first race since January – in which she enters as the top seed by personal best time.

“The thing with glandular fever is there is nothing you can do but rest,” she said.

“I have a good week of training and then the fatigue will hit, or one week will be a write-off. It is something I have to be gentle with myself.

“I have compared it to the start of Covid where there was nothing you can do. I just have to accept it.”

Anna Hopkin, who has already won the 50m title in London this week, was the fastest qualifier for the final on Sunday in an impressive 53.68secs – marginally outside of the qualification time of 53.55secs.

Eva Okaro was second in 54.60secs, followed by Anderson’s 55.01secs.

Only two swimmers can race in each event in Paris with the winner of each British final taking an automatic place, subject to them making the qualification time.

The British coaches can use their discretion for the second pick, giving Anderson a chance of making the event even if she does not finish in the top two.

Olympic champion Tom Dean was the fastest qualifier for Sunday’s showpiece race – the 200m freestyle.

The race has been billed as the most competitive domestic race on the planet with 2023 world champion Matt Richards and Olympic silver medallist Duncan Scott, plus James Guy – who won relay gold with the trio in Tokyo – also through to the final.