Heavily armed plain-clothes police will patrol the UK's bank holiday festivals this weekend in a sign of increased tensions over another terrorist attack.

Elite Special Forces troops will be in civilian clothing and mingle with crowds carrying hidden weapons, according to security sources quoted by the Mail on Sunday.

'They will be casually dressed with their weapons hidden beneath their clothes. They won't be in traditional military vehicles either,' the source said.

'Recently, a company of Special Forces Support Group troops completed a live firing exercise using Civilian Armoured Vehicles and they will be called upon.

'These are regular cars but with special adaptations such as armour-plated doors and reinforced bullet-proof windows to assist them in an urban close quarter battle scenario.'

Among the events named by the Times newspaper as possibly receiving additional security was Greenbelt – a Christian music and arts festival at Boughton House in Northamptonshire.

A spokeswoman at Greenbelt neither confirmed or denied whether there would be armed police but said: 'We constantly review plans to keep festivalgoers safe, in line with many other events and venues in this country.

'We work in accordance with guidance from NaCTSO and other appropriate agencies. In line with their recommendation we do not release detailed information about our security plans, nor have we ever.'

It comes as the UK's threat level issued by MI5 remains at 'severe' after attacks on London Bridge, Manchester Arena, Westminster Bridge and Finsbury Park this year.

Christian Today also contacted Soul Survivor, a major Christian youth festival which includes the Bank Holiday weekend but has not received a response.

Earlier this summer Keswick Convention, a Christian three-week gathering in the Lake District, revealed armed police to keep festivalgoers safe.

Keswick Ministries spokeswoman Jutta Devenish said: 'We have been taking advice from local police over a long period of time.

'They said that as these actions have been recommended on a national basis, they advise us to do the same.

'We have not been given any information that we are likely to be a target, we just want to reassure people in the town and conventioners.

'Armed police will also be on patrol in the town to reassure people and Keswick Ministries has hired its own professional SIA trained security team.

'We have only had one enquiry about security for this year's event, which is pretty amazing.'