The level of threat from international terrorism to Britain has been raised from substantial to severe, Theresa May, the home secretary, has announced.

This means that an attack is "highly likely", though May said there is no evidence to suggest an attack is imminent.

She said the decision to raise the official threat level is "related to developments in Syria and Iraq, where terrorist groups are planning attacks against the west".

The home secretary said some of the plots were likely to involve foreign fighters who had travelled to the Middle East from Britain and Europe to take part in the conflicts there.

Her announcement is expected to be followed shortly by a Downing Street statement by David Cameron.

The official threat level assessment last stood at "severe" more than three years ago when it was reduced to "substantial" on 11 July 2011. Severe is the fourth level in the five-tier system of alert categories that have been used by the government since 2006 to warn of terrorist activity. The only higher level is "critical", which is used when an attack is expected imminently.

May said the decision to raise the threat level had been taken by the Joint Terrorism Analysis Centre: "We face a real and serious threat in the UK from international terrorism. I would urge the public to remain vigilant and to report any suspicious activity to the police," she said.

Assistant Commissioner Mark Rowley, the national policing lead for counter-terrorism, said the change in the threat level meant that established planning mechanisms across the police service had been activated: "This will lead to enhanced prevention and preparedness. From this afternoon we will begin to increase our levels of visible patrols and implement other security and protection measures. We will build on existing community relations to provide reassurance and seek their support and assistance in keeping the UK safe."

A range of factors lie behind the change of assessment of risk by the security services and counter-terrorism chiefs.

Anyone with information is urged to contact the Anti-Terrorist Hotline on 0800 789 321.

The national threat levels explained

There are five levels of threat used by the JTAC. These are:

• Critical - an attack is expected imminently

• Severe - an attack is highly likely

• Substantial - an attack is a strong possibility

• Moderate - an attack is possible but not likely

• Low - an attack is unlikely