The prime suspect in the killing of WPc Yvonne Fletcher will not be prosecuted after police were blocked from using key evidence on “national security” grounds, it has emerged.

Saleh Ibrahim Mabrouk, a former education minister under Colonel Muammar Gaddafi, was arrested in 2015 in connection with the murder of WPc Fletcher, shot dead while policing a demonstration outside the Libyan embassy in 1984.

But Mr Mabrouk, who lives in the south of England, has been told by police that the case against him will not proceed “at this time”, despite Scotland Yard’s belief it has enough evidence to bring a prosecution.