Civil liberties and privacy campaigners say that more than £3.9 million has been spent by public bodies across the country in the last two years on paying private investigators for surveillance work - including snooping on their own staff.
The Big Brother Watch study found 27 local councils nationally had employed private firms to undertake surveillance - ten of which are in London.
Hammersmith & Fulham
Kensington & Chelsea
More top news
The ComRes survey for ITV News and LBC showed the Labour candidate increasing his overall lead over Conservative rival Zac Goldsmith.
The latest poll for ITV London and LBC shows Sadiq Khan leads Zac Goldsmith by 45% to 36% on first preference votes.
Corbyn was unveiling a poster less than a mile away as Khan gave his final campaign speech to party supporters in London.