London's New Year's Eve security measures will involve 3,000 police officers amid fears of Isis attacks

Armed units will be among more than 3,000 police officers being deployed to protect London’s New Year’s Eve celebrations amid concern over the threat of Isis-inspired terror attacks.

Security plans have been “adjusted” in the wake of the lorry attacks in Berlin and Nice, which followed previous shooting massacres and bombings in Brussels and Paris.

Scotland Yard said around 3,000 of its officers will be on duty across central London on 31 December, both inside and outside of ticketed areas for viewing fireworks along the Thames.

Armed British Transport Police officers will also carry guns on London Underground trains for the first time, having previously been stationed inside Tube stations, as security is increased.

Detective Superintendent Phil Langworthy said there were “both over and covert” measures in place to protect hundreds of thousands of people expected to gather for the Mayor's firework display.

"Clearly we have been looking at what has happened around the world in terms of Berlin, Nice, etcetera, and have adjusted our plans and continue to adjust our plans,” he told the Press Association.

"We police around 3,500 large events every year including New Year's Eve and we meticulously plan those events - we have meticulously planned New Year's Eve - and we look at our tactics and we look around the world and adjust our tactics if need be.

"We have a very extensive planning period, we plan for many months, pretty much we start planning since the last event."

Det Supt Langworthy said there was ”no specific intelligence“ for an attack on the end-of-year event, but added: "I would encourage people on the night if they see anything suspicious or have any concerns to come and speak to one of the police officers or stewards who will be on duty."

Isis has used its propaganda outlets to call on supporters to launch attacks in countries, including Britain, that are supporting air strikes by the US-led coalition on its territories in Iraq and Syria.

The group has released detailed guidance on launching lorry attacks, car rammings, stabbings and manufacturing homemade explosives to inflict maximum casualties and terror.

Its supporters have additionally released unofficial propaganda calling for New Year’s Eve and the festive period to be made into “bloody horror movies” for “disbelievers” in the West.

Heightened security precautions are expected to include concrete barriers designed to shield crowds from attempted car and lorry attacks.

They have been deployed around Christmas markets across Germany following the Berlin attack on 19 December, when a Tunisian Isis supporter ploughed a lorry into crowds next to the Kaiser Wilhelm Memorial Church, killing 12 people and injuring 50 more.

It followed the Nice attack in July, when another Tunisian man mowed down crowds celebrating Bastille Day, killing 86 people and injuring at least 400.

Theresa May urged Britain to celebrate the festive season without being “cowed by terrorists” following the latest atrocity, saying the security services were continuing their work after foiling several previous plots.

Det Supt Langworthy said police wanted to ensure people stayed safe and have a good time, adding: “Officers have been planning for several months for New Year’s Eve, and that plan remains under constant review. This is not as a result of any specific intelligence.”

Major cities around the world are putting security measures in place, with rubbish lorries filled with sand being used to close off streets around New York’s Time Square. Fireworks at the Eiffel Tower in Paris have been cancelled as a precaution.