Armed French servicemen are being dropped by helicopter on to ferries in the English Channel in a bid to prevent terrorist attacks.
Sea marshals carrying guns are arriving by helicopter as French-flagged vessels leave English territorial waters on their way to France.
The first patrol took place Monday, when three French sea marshals arrived on the Brittany Ferries vessel Mont St Michel by helicopter.
They then proceeded to patrol the bridge and passenger areas of the vessel.
Speaking to CNBC Thursday, a spokesperson for Brittany Ferries said the dramatic boarding of ships in mid-journey was due to maritime law.
"There is no agreement in place for French armed military personnel to be allowed in British territorial waters," the spokesperson said.
"The urgent aspiration from the French government is that there will be an agreement struck between governments so that sea marshals can patrol the ships for the full duration [of the crossing]," said Nigel Wonnacott to CNBC.
In a statement issued by the U.K. Home office and seen by CNBC, a spokesperson said the British government "work extremely closely with our French counterparts" and that security arrangements were "under constant review".
French authorities told the BBC Wednesday that no timeline had been set for any agreement to be reached.
Wonnacott said passengers had, by and large, accepted the sea marshals' arrival.
"The world has changed. Had this happened a few years ago I think there would have been alarm."
Brittany ferries said it was unclear how long the random exercises would continue for.
The Danish ferry company DFDS is the only other passenger ship that offers channel crossings under a French flag.
One of its ships was boarded Wednesday night on a sailing from Newhaven in England to Dieppe in northern France.