Armed police officers are patrolling on board trains in and out of London for the first time.

The London Underground network has had armed officers on guard since December, but this is the first time they will be on trains travelling to and from the capital.

British Transport Police (BTP) announced the measure in a bid to "disrupt and deter criminal activity" on the rail network after the UK terror threat level rose to critical in the wake of the Manchester attack.

BTP Chief Constable Paul Crowther said: "In light of the dreadful events over these last few days, and the increase to the national threat level, nothing should be considered too trivial to report and any information - regardless of whether you feel it is significant or not - may be important to all of us.

"The cowardly acts of terrorists will never weaken our resolve."

He added that the deployment of armed officers will be focused on rail routes serving "big city locations" but will not be limited to those.

He said: "The aim is for it to be unpredictable and widespread across the network, to create a deterrent and an immediate response to terrorists who may be thinking of (attacking) the transport network."

Among the first to be deployed were a group of four officers who boarded a Virgin Trains service at London Euston travelling to Birmingham New Street shortly after 2pm yesterday (May 25).

Mr Crowther said: "Since the devastating events in Manchester on Monday evening, our force has radically increased the presence of our officers nationwide.

"By having firearms officers on board trains we're ensuring that trains remain as safe as possible for passengers.

"Our patrols will be highly visible and passengers should feel comforted by their presence."

Mr Crowther urged passengers not to be alarmed by the presence of armed officers.

"These are normal officers," he said. "They are very approachable. They are there to engage with people, to talk to them about any concerns they've got. I'd really encourage people to approach them."

Paul Plummer, chief executive of the Rail Delivery Group, representing train operators and Network Rail, said: "It is absolutely right that the British Transport Police takes all necessary steps to keep the travelling public and visitors to stations safe.

"Together with the police, everyone on the railway will be remaining vigilant."