The man responsible for overseeing the conduct of the UK's intelligence services has said that MI5 lying to multiple courts while defending a violent neo-Nazi spy is as "serious as anything" he has ever dealt with.

"Oversight depends on candour," said investigatory powers commissioner Lord Leveson. "Failings of candour undermine the entire basis of oversight and accountability."

His office, the Investigatory Powers Commissioner's Office (IPCO), investigated how MI5 gave false evidence to three courts concerning the abusive informant known as agent X, as first exposed by the BBC.

IPCO's report, released earlier this year, found one senior officer lied repeatedly while another misled his own colleagues and lied about what he was told.

It also criticised other senior spies and found "serious and systemic failures in MI5's conduct" throughout the case.

MI5 said it accepted the report's findings "without reservation" last month .…