Britain's MPs have been told to increase security in their homes and constituencies amid fears they and their staff could be singled out for an attack by Islamic State militants.

Islamic State (ISIS) has a political assassination unit aimed at government officials and the Syrian bombing campaign has moved the spotlight on to the UK now, according to Sunday Express.

With 300 suspected Islamist terrorists returning to the UK and now free on the streets, security chiefs have warned of a direct threat.

A security expert told the newspaper: "MPs are legitimate targets in the eyes of terrorists. Ministers who are in Metropolitan areas, where there are UK jihadists who have returned from Syria, are right to remain vigilant".

"The definition of a terrorist attack is the threat of violence or actual violence against someone to force the government of the day to change their principal policies. Who makes the decisions? The MPs do," the expert said.

"They are members of the establishment seen as being more responsible for the bombing of Islamic State targets in Iraq and Syria than RAF personnel. Those MPs who recently voted in favour of bombing Syria will also be more at risk," he added.

In 2010, Labour MP Stephen Timms was stabbed in his constituency office in east London by 21-year-old Islamist extremist Roshonara Choudry because he voted for the Iraq war.

She was jailed for life for attempted murder. Some MPs are thought to have installed panic buttons in their constituency offices. A House of Commons spokesperson said it was policy not to discuss security matters.

Scotland Yard is currently investigating around 600 terror cases relating to Syria and Iraq.