The police helicopter took to the skies and two men were arrested on separate roads nearby.

After the arrests, police searched the area thoroughly and found the grenade on top of the garden wall.

A Merseyside Police spokeswoman said: “The Army's bomb disposal team were called and the area cordoned off for the public's safety.

“The device was removed and made safe and is currently undergoing forensic examination.

“Selworthy Road is cordoned off at its junction with Sandringham Road and high-visibility patrols have been stepped up in the area to reassure the public.”

The houses of Mr Ball and Riley have previously been shot at, and businesses – including Shorrocks Hill nightclub, in Formby, and the Cabbage Inn, in Netherton – have been firebombed.

There have also been attacks on relatives, including shots fired at homes including that of Mr Riley’s father and Mr Ball’s partner, in a terror campaign that began at the start of the year.

Last week, police raided the business address of Mr Ball in connection with the assault of a teenager, in Bootle.