A YouGov poll for the Evening Standard places Dame Tessa narrowly ahead of Diane Abbott with the backing of 16% of supporters.
The poll reverses a June survey which placed Ms Abbott ahead by 17 per cent to 14 per cent.
Among the wider public Dame Tessa is the choice of 12 per cent of poll respondents with Ms Abbott again in second place with 8 per cent.
A possible Jowell candidacy is backed by 20% of Liberal Democrat supporters, 13% of UKIP voters and 8% of Tories, suggesting the former cabinet minister has the cross-party support needed to win the Mayoralty.
The poll also reveals that Baroness Doreen Lawrence has overtaken MPs David Lammy and Sadiq Khan and is now in third place with the backing of 7 per cent of party supporters.
Christian Wolmar, the only declared candidate, has the backing of 2% of party members.
More than a third (37%) of respondents say don’t know who to back.
