Four men have been arrested on suspicion of plotting a terror attack in the UK as part of an investigation by MI5.

Officers from the Metropolitan Police's counter terrorism command detained the suspects during raids on their homes.

The men, aged between 18 and 27, remain in custody while searches continue at five homes in east London and a nearby business address.

A spokesperson for Scotland Yard said they were detained “on suspicion of being concerned in the commission, preparation or instigation of acts of terrorism”.

“These arrests were pre-planned as part of an ongoing investigation by the Met's Counter Terrorism Command and MI5,” he added. “The arrests are linked to activity in the UK.”

Investigators gave no further details on the operation, which follows a series of stings targeting alleged Isis supporters in Britain.

In pictures: Westminster attack Show all 9 1 /9 In pictures: Westminster attack In pictures: Westminster attack An air ambulance lands after gunfire sounds were heard close to the Palace of Westminster in London PA wire In pictures: Westminster attack MPs wait until the situation is under control in Westminster. 'The alleged assailant was shot by armed police,' David Lidington, leader of the House of Commons, told the house. BBC News In pictures: Westminster attack Crowds gather in Westminster after shooting incident, which police are treating as terror attack BBC News In pictures: Westminster attack Police were also called to an incident on Westminster Bridge nearby AP In pictures: Westminster attack Early reports indicate the car, which mounted the pavement on Westminster Bridge and mowed into around a dozen people, was the same vehicle which then rammed into the railings of the Palace of Westminster, just around the corner Reuters In pictures: Westminster attack Security sources described the suspected assailant as a middle-aged Asian man, who is understood to have left the car before attacking a police officer with a seven-to-eight inch knife PA wire In pictures: Westminster attack Police have asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident AP In pictures: Westminster attack One woman has died and a number of others, including the police officer, have been hurt, according to a junior doctor at St Thomas' Hospital Reuters In pictures: Westminster attack At least three gun shots were heard by those inside Westminster, and proceedings in the House of Commons have been suspended AP

Security has been intensified in the capital since alleged Isis supporter Khalid Masood killed five people in a car and knife attack in Westminster in March.

Last month, a man was arrested while carrying several knives in Whitehall. Khalid Mohammed Omar Ali, 27, has since been charged with preparing a terrorist act and two explosives offences dating back to time spent in Afghanistan in 2012.

In an unconnected case that emerged on the same day, three women have been charged with preparation of a terrorist act and conspiracy to murder following a police raid in Willesden that saw one of the women shot.

Rizlaine Boular, 21, her mother Mina Dich, 43, and Khawla Barghouthi, 20, remain in custody ahead of a hearing at the Old Bailey on Friday.

Separately, 24-year-old Sabbir Miah pleaded guilty to spreading Isis propaganda videos on Tuesday.

British security services have prevented 13 potential terror attacks since June 2013, Assistant Commissioner Mark Rowley revealed.