Khan has the backing of 31 per cent of London voters, compared to 26 per cent supporting Goldsmith, according to a poll conducted by Opinium for the Evening Standard.
According to the pollsters, Khan is also set to net twice as many second preference votes as Goldsmith, putting him on track to win the run-off by a margin of 55-45.
The results indicated, however, that the race is still relatively open, with a quarter of Londoners saying they have yet to make up their minds and the margin between the leading candidates much closer in outer London.
Opinium found little support for candidates from other parties. Ukip’s Peter Whittle was slightly ahead of Green Party candidate Sian Berry and Liberal Democrat candidate Caroline Pidgeon, though all three had the support of about two per cent of the electorate each.
Respect Party candidate George Galloway commanded less than one per cent of voters' support.