London: Cross-Channel ferries and the Palace of Westminster have been identified as facing an increased risk of terrorist attack in the wake of the atrocities in Paris.

Ministers have highlighted passenger ferries as a weak link in Britain’s defence against Islamist terrorists, amid fears they could hijack a ferry in the Channel and commit slaughter before security forces could reach it.

Meanwhile, MPs and staff have been issued with new guidance on what to do in the event of a terrorist attack on the Palace of Westminster. They have been told: “If you hear gunfire, move away immediately but consider barricading yourself and others in a room if danger is close.”

On Saturday, a security expert called for the introduction of sea marshals — armed officers — to be introduced on ferries, as well as on trains, to protect passengers in the event of a terrorist attack. Armed air marshals were installed on transatlantic airliners after the 9/11 attacks on the US in 2001.

A senior government source admitted that security at ports and on ferries was a major concern. The source said: “Ports are something we have started to look at.

“Security at airports has been tightened in recent years and having done that there is a displacement affect. The more you make one area of attack less attractive, you inevitably make another more attractive.”

The source refused to discuss specific measures, but added: “It [the introduction of sea marshals] would not be unreasonable. It is not something we would give details on but we are looking at maritime security.”

Cars are not routinely searched before going on ferries, although suspicious vehicles can be picked out. Security experts believe trains and football stadia would also continue to be a prime target for Isil.

A “lone wolf” terrorist gunman targeted a train travelling between Belgium and France in August but the attack was foiled by two off-duty US servicemen on holiday in Europe.

Olivier Giutta, managing director of GlobalStrat, an international security consultancy, said he was convinced Daesh will continue to target trains until they succeed. He said football matches would also continue to be under threat, despite the attack on the Stade de France having been thwarted.

Professor Anthony Glees, director of the Centre for Security and Intelligence Studies at the University of Buckingham, said it was time to consider security marshals on ferries and trains.

He added: “Now that there is a strong probability that there are roving terrorists in Europe, possibly even in the UK, ready to carry out attacks against any kind of target, we need to consider new security measures to protect our transport system.

“The terrorists will have thought against targeting airports such as Heathrow and Gatwick, where security is strong, but trains — where there was a failed attack in France a few weeks ago — and ferries are vulnerable.

“I would like to see people in uniform on these services who can call in a rapid-reaction force in case of attack. If this is not possible then they should have access to securely stored weapons on board ships or on trains. Transport is more vulnerable to the ‘lone wolf’ with a Kalashnikov.”

The warning to MPs and staff in the Palace of Westminster was sent on Monday, but had to be resent after a technical problem meant it failed to reach some email addresses, prompting concern from some MPs. Circulated by Black Rod, the Serjeant at Arms and Parliament’s security director, it said: “The security situation in Parliament is being kept under close review following the terrorist attacks in Paris.”

It added: “In the event of an incident involving firearms or other weapons, the police will take operational control. MPS [Metropolitan Police Service] firearms officers are already deployed on the Parliamentary Estate and will respond to such incidents. “Police instructions should be complied with immediately.”

The Telegraph Group Limited, London 2015