An ingenious thief has swiped almost a quarter million dollars on a flight in the Caribbean after sneaking into the cash-laden cargo hold via the toilet, police say.

A Brink's security employee placed three sacks of cash containing a total 1.2 million euro in the hold of the Air Antilles plane before it headed from the French island of Guadeloupe to the Franco-Dutch island of Saint Martin.

The security guard took his seat on the ATR-42 turboprop plane but when the flight landed 40 minutes later it was discovered that 172,000 euro ($236,000) were missing from the sacks.

Police are seeking a man who complained he felt ill and spent most of the journey in the toilet. In fact, he was removing panels to gain access to the hold in the rear of the plane.

Shortly before landing, the unnamed man - who was travelling with a woman who appeared concerned about his health - asked a hostess for an ambulance to meet him on the tarmac, witnesses said.