The Met statistics show that 3,821 emergency 999 calls went unanswered on Monday August 8, the third day of last month’s rioting in the capital.

Three quarters of non-emergency calls to police were also abandoned. It also took an average of 75 seconds that day for operators to pick up the 999 calls that were answered – around 10 times longer than the time taken on a normal day.

During the evening, the waiting time rose to an average of 100 seconds. Met chiefs said the problems were caused by the “deluge” of calls received and that subsequent checks had found “no issues of concern” over the 999 callers who had given up trying to get through.

Caroline Pidgeon, a Liberal Democrat member of the Metropolitan Police Authority, who obtained the figures, warned, however, that members of the public could have been put at risk and urged reforms to prevent similar problems occurring in future, saying: “Although the events of August 8 were exceptional it cannot be right that almost one in five people couldn’t get through via the 999 emergency number and simply gave up trying to contact the police.”

The figures show that the Met received 20,940 separate 999 calls on August 8 of which 3,821 – or 18.2 per cent – were abandoned after the caller either gave up waiting for a reply or rang off before sufficient details had been taken.

A total of 15,984 calls to the Met’s non-emergency 101 number were also abandoned on the same day.

The following day, when Scotland Yard’s decision to flood the streets with 16,000 officers halted the disorder, there were only 119 abandoned 999 calls, although there were still problems with 101 calls.

The Met said that many calls were abandoned rapidly and that some were not “emergencies or genuine calls for police assistance” and that the number of unanswered calls was increased by the tendency of some people to repeatedly ring, hang up, and then redial.

The Met said that because of the “exceptional demand” placed upon operators during the riots, priority had been given to 999 callers, leading to a reduced service for those ringing 101.