An Australian triathlete was mistaken for an illegal immigrant by British residents and held up by border authorities when set to swim the English Channel, local media reported.

John van Wisse was attempting to set a world record on an ultra-triathlon from London to Paris when he was swooped on by local police and the British Coastguard, the Dover Express reported.

"It looks like a couple of people called thinking it was illegal immigrants coming ashore because some people were seen jumping off a boat," a police spokesperson told the newspaper.

"The Border Force were down there and the Coastguard.

"But it was a Channel swimmer."

Police saw "the funny side" and allowed van Wisse to proceed after a short delay on the second leg of the triathlon, having already run the 87-mile first leg from London's Marble Arch to the Kent coast.

The third and final leg is an 181-mile bike ride from Calais to the Arc de Triomphe in Paris.