A spy chief yesterday denied MI5 was “seriously unwise” to consider closing an investigation into one of the London Bridge attackers just weeks before his murderous rampage.

The Security Services were monitoring Khuram Butt from mid-2015 until the point he and two accomplices killed eight people in a knife and van attack on June 3 2017.

The 27-year-old was subject to “extensive coverage” by a covert surveillance team, but their work was twice suspended due to an unprecedented wave of terror plots, the Old Bailey heard.

Speaking anonymously at the inquests into the deaths of eight victims, a senior MI5 officer known only as Witness L said it was the most “alarming” period he had faced in three decades.

The first suspension of the investigation into Butt came in February 2016 following a spate of outrages in mainland Europe, followed by a month-long pause from March to May 2017.

The second suspension, covering the immediate wake of the Westminster Bridge terror attack, was justified due to "the unprecedented level of threat which we were facing and therefore the pressure on our resources", he claimed.