Australian John van Wisse has encountered an unexpected obstacle in his ambitious ultra-triathlon bid from London to Paris - the British border patrol.

The Melbourne athlete, who is on track to break the record for one of the world's toughest sporting challenges, was briefly mistaken for an illegal immigrant as he prepared to begin the swim leg across the English Channel on Wednesday.

John Van Wisse. Credit:Stephen McKenzie

Police and coast guards were called to Dover's Shakespeare Beach by concerned locals around 8.30am but quickly saw the funny side when they realised what they were dealing with.

A Kent Police spokeswoman told the Dover Express it appears some people called police, thinking it was illegal immigrants coming ashore after several people were seen jumping off a boat.