Armed police officers are patrolling on board trains nationwide for the first time.

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.

Armed officers have been patrolling on the London Underground network since December, but this is the first time they will travel on trains outside the capital.

The force urged passengers not to be alarmed by their presence.

BTP Chief Constable Paul 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.

"Please do speak to them if you have any concerns at all.

"This is part of our national mobilisation plan to deal with the current and ongoing threat.

"It is important to note that we do not have any specific intelligence in relation to train services but are taking this action to ensure we can protect and reassure the public."

Since the Government activated Operation Temperer, BTP has benefited from additional firearms officers from the Ministry of Defence Police, who will remain at stations in London.

Mr Crowther went on: "I want to take this opportunity to remind everyone to stay calm, be vigilant, and if you spot anything at all which gives you cause for concern, let us know.

"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.

"We will continue to work with police forces nationally to ensure that the travelling public remain safe and reassured."

Manchester Victoria station, which is attached to Manchester Arena where the blast happened, was closed to allow a forensic search of the area and has not yet reopened.