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."

Berlin Christmas market lorry attack Show all 18 1 /18 Berlin Christmas market lorry attack Berlin Christmas market lorry attack Several people have been killed after a lorry drove into crowds at a Christmas market in Berlin REUTERS/Fabrizio Bensch Berlin Christmas market lorry attack 'At least nine' people have been killed and more than 50 injured. AP Berlin Christmas market lorry attack Emergency Services rush a Berlin market victim to an ambulance Associated Press Berlin Christmas market lorry attack Police cordoned off the square at Kaiser Wilhelm Memorial Church following the incident REUTERS Berlin Christmas market lorry attack Rescue workers inspect the lorry that crashed into a Christmas market close to the Kaiser Wilhelm memorial church in Berlin EPA Berlin Christmas market lorry attack Emergency crews inspect the lorry that ploughed into a Berlin Christmas market, killing at least nine people AFP Berlin Christmas market lorry attack Fire crews attend the scene of the attack AFP/Getty Berlin Christmas market lorry attack Armed police secure the site of a lorry attack at a Christmas market in Berlin REUTERS Berlin Christmas market lorry attack Crushed debris is visible beneath the wheels of the vehicle REUTERS Berlin Christmas market lorry attack An injured man is pushed to an ambulance REUTERS Berlin Christmas market lorry attack Medics attend an injured person after the lorry attack which killed at least nine and injured more than 50 people AFP/Getty Berlin Christmas market lorry attack Firefighters examine the lorry which was rammed into a Berlin Christmas market REUTERS Berlin Christmas market lorry attack A person is carried into an ambulance REUTERS Berlin Christmas market lorry attack View of the lorry that crashed into a Christmas market in Berlin, killing at least nine and injuring at least 50 people AFP/Getty Berlin Christmas market lorry attack Rescue workers push a person on a stretcher to an ambulance Getty Berlin Christmas market lorry attack Firefighters assess the damage after the lorry rammed the Christmas market, killing 'at least nine', and injuring more than 50 people AP Berlin Christmas market lorry attack Firefighters stand beside a toppled Christmas tree at the site of the suspected terrorist attack in a Berlin Christmas market AP Berlin Christmas market lorry attack Damaged stalls at the scene of the incident at a Berlin Christmas market where at least nine people have been killed EPA

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.

Angela Merkel reacts to shooting of Berlin market attack suspect

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.”