Image copyright EPA Image caption French police arrived by helicopter to patrol the Mont St Michel

Armed French police have begun patrols on cross-Channel ferries in a bid to prevent terrorist attacks.

The scheme started on Monday, but with no agreement yet in place for French police to patrol vessels in UK waters, they arrived by helicopter and boarded a ferry as it approached France.

The two governments are in talks about French officers patrolling for full crossings, France's marine police said.

The Home Office said security plans were under "constant review".

French authorities told the BBC no timeline had been set for an agreement to be reached.

'Terrorist threat'

Speaking to the AFP news agency, French marine police spokesman Lieutenant Pierre-Joachim Antona said a "permanent unit" had been deployed since Monday to carry out high-visibility patrols on passenger ferries.

"The marine gendarmes will carry out patrols, which will be random but regular, with the aim of securing these vessels against the terrorist threat," he said.

The first patrol took place on Monday, when three French sea marshals arrived on the Brittany Ferries vessel Mont St Michel by helicopter.

A Brittany Ferries spokesman said the vessel left Portsmouth for Caen in France at 14:45 BST and, in a "pre-scheduled security exercise", the marshals were flown on to the boat at 17:30.

"They then proceeded to patrol the bridge and passenger areas of the vessel," he said.

He added: "Passengers were informed via announcements in English and French before the exercise took place.

"Access to outside decks was not allowed at the time of the helicopter's arrival."

'Treading carefully'

Professor Andrew Serdy, a maritime law expert at Southampton University, said French police have "no jurisdiction" in UK territorial waters, except by agreement with the British government.

But he said airlifting officers showed an "abundance of caution" - and suggested France may be "treading carefully" until a deal with the UK is agreed.

Outside of UK and French territorial waters - which stretch up to 12 nautical miles from the countries' coasts - Prof Serdy said jurisdiction would depend on where a vessel was registered.

Image copyright Reuters Image caption Security has been heightened in France after a series of deadly attacks

Brittany Ferries' 10 vessels are all "French-flagged", meaning French police could operate on them outside territorial waters.

It is not clear whether the French police's plan includes non-French vessels, such as UK-flagged ferries arriving in France.

The UK Home Office did not comment on the talks with French officials.

"We work extremely closely with our French counterparts on matters related to security and counter-terrorism," a spokesman said.

"We keep security arrangements, including those related to maritime security, under constant review."

The French police plan comes in the wake of several attacks in European countries including France.

Last month there were lengthy delays for UK passengers heading to the port of Dover due to increased security checks at French border posts.