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

British Transport Police 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.

Armed tansport police officers on board a Virgin train from Birmingham to London Euston

The move to have armed officers on trains comes in the wake of an increased terror threat level

It is the first time armed officers have patrolled trains outside of the capital

BTP Chief Constable Paul Crowther said the deployment of armed officers will be focused on rail routes serving 'big city locations' but will not be limited to those.

'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,' he said.

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 on Thursday.

Mr Crowther went on: '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.'

The BTP introduced armed patrols of London train stations in February 2012 in response to the threat of a Mumbai-style marauding gun attack.

Since then police firearms officers have appeared at other major stations.

The country is on a heightened threat level as police attempt to track down any of the Manchester suicide bomber's associates

Officers patrol Birmingham New Street as armed police were deployed around the country

The head of British Transport Police said armed police would be travelling on all lines

Specialist Operations officers wait to board a Virgin train to Birmingham New Street today

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.'

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 insisted that BTP does not have any specific intelligence in relation to a threat to the rail network, but called for the public to remain vigilant.

He 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.'

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.'

The move comes after soldiers were drafted in to patrol the streets of London with police

A soldier stands guard at Downing Street in London today as part of the new measures

Almost 1,000 military personnel are under the control of police as part of Operation Temperer

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.

Armed police carried out a series of raids today, believed to be a hunt for the bomb factory.

Eight people remain in custody in connection with Monday's attack. Armed police today chased and arrested a man in Nuneaton, Warwickshire and bomb disposal experts rushed to the Hulme area of Manchester, where armed officers pinned a man to the floor. The home of a man arrested in Wigan yesterday is also being searched.

In response to the heightened threat, the Government has activated Operation Temperer, providing up to 3,800 troops to support the police in their security operations.

Home Secretary Amber Rudd said 984 military personnel were being deployed around the country, which the Metropolitan Police said had freed up 1,000 armed officers to carry out patrols.