The swim is expected to take about 24 hours, covering almost 40 miles, and follows his previous challenges which include swimming the length of the Jurassic Coast, becoming the fastest person to swim around the Isle of Wight and the first person ever to swim around Grand Cayman. Each swim has raised money to help clean up the world’s oceans, and this one is no different. Every penny donated will be split between Project Planet Earth and Healthy Seas.

With the Ithaca swim, Oly will face the potential danger of jellyfish, so will have spotters on his support boat looking out for them.

The challenge has been made possible by recruitment company Green Folk Ltd based in Hook, Hampshire, which has funded Oly during his training and is covering his costs.

In addition, Oly has received help in planning from Adriana Eyzaguirre who runs a kayak tour company on the island, while Kerry Jamieson of Argos Animal Welfare will be helping to observe the swim and ensuring the rules are followed. Even the mayor of Ithaca has offered to help out with free accommodation.

Oly said: “The sea there is around the island about 28 degrees which compares with 17 degrees in the water where I’ve been training.

“The main dangers are sunburn, exhaustion and jellyfish, which at certain times of the year are found in large numbers. But I have a really professional team and hopefully we can raise plenty of money for the two good causes.

“I am lucky in that I can combine my hobby, which is swimming, with my passion, which is helping better protect the oceans.”

Oly spends his time clearing plastic from Dorset’s beaches as well as giving talks to schoolchildren about plastic pollution.

The swim is planned to take place on Monday 4 September although, if conditions aren’t right, it could be delayed for a few days.

There will be a tracker on the www.projectplanet.earth website and the link to make donations is https://gofund.me/a2c63c3d