A Brisbane teenager has become the second-youngest person to swim the English Channel twice in one day, swimming 68 kilometres in 22 hours to make it happen.

Key points: Brianna Thompson took 22.5 hours to swim from England to France and back

Brianna Thompson took 22.5 hours to swim from England to France and back The 17-year-old says jellyfish stings were one of the worst parts of the endurance swim

The 17-year-old says jellyfish stings were one of the worst parts of the endurance swim She cannot eat food for three days because the saltwater stripped a layer off her tongue

Brianna Thompson, 17, lost a layer of her tongue and suffered from severe leg cramps and jellyfish stings in the swim from England to France and back — a feat of endurance only achieved by 20 others.

Her Sunshine Coast-based training partner, Sam Penny, said Brianna also had a swollen throat and faced the constant threat of hypothermia from swimming in the 16-degree Celsius water.

She finished the swim at Samphire Hoe, near the English town of Dover, about 11:35am on Wednesday.

In a statement, Ms Thompson said the worst part had been the "jellyfish stings and massive piles of floating seaweed".

"When you are swimming for hours, these things give you a big fright," she said.

"The jellyfish hurt for ages. They really aren't fun."

She said despite being back on dry land, she was too sore to sleep or sit in a chair.

"I'm in agony," she said.

"It will be days before I can lift my arms above my head."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 13 seconds 13 s The 68km swim took Brianna Thompson just over 22 hours to complete

The saltwater stripped a layer off Brianna's tongue, and she said she would be unable to fulfil her pizza craving for at least three days.

The strong tides in the channel meant she was not able to stop to eat, so she received food through a tube.

"I could feel my throat swelling up during my swim and each feed became more painful," she said.

Ms Thompson was also only allowed to spend a minute on the other side of the channel before turning around and swimming back.

"The rules are you have to clear the water and hop back in immediately," Mr Penny told ABC Sunshine Coast.

"But you wouldn't want to hang around too long. The air temperature is what cools you down."

The Channel Swimming Association has strict guidelines for swimming the channel, including that it must be done in "traditional swimmers, goggles and a cap".

The teenager trained on the Sunshine Coast before completing the double crossing. ( Facebook: Brianna's Channel Swims )

"As my body began to shut down towards the end, I could feel the cold creeping into my body," Ms Thompson said.

"But then the adrenaline of finishing kicked in the closer I got and I knew I was safe from hypothermia."

An expensive and difficult swim

Ms Thompson trained in Brisbane with her English Channel world record holder coach, Trent Grimsey, on the Sunshine Coast on weekends.

Ms Penny said the teenager had no sponsors and her mother had worked two jobs to raise the $40,000 cost of preparing for and completing the double crossing.

"There are only 20 people in history who have made it, and she is the first person in three years to finish it," Mr Penny said.

"People try every year to do the double crossing and can you believe a 17-year-old training on the Sunshine Coast absolutely smashed it?"

The Boondall teenager is also the youngest person to have completed three crossings in 12 months after her first single English Channel crossing in September 2018.