LONDON (REUTERS) - Britain thwarted a Sydney-style 'lone wolf' attack just days before it was due to happen, London's top policeman said on Wednesday, saying his force had foiled five terror plots in the last four months alone.

Britain raised its terrorism alert to the second-highest level in August and last month said it was facing the biggest terrorism threat in its history because of radicalised Britons returning from fighting in Syria and Iraq.

"They've been very close to going and hurting somebody, badly, or killing them... within days," London's police chief, Bernard Hogan-Howe, said in an interview on LBC Radio.

"In terms of plots over this last four months really, it's probably five. And we've arrested I think it's about 35 people."

Hogan-Howe said the activity had put a strain on resources and that more funding was needed to address backlogs in retrieving digital evidence to identify such plots.