The poll shows that Dame Tessa is now the preferred choice of 40% of all Londoners – up from 32% in February.
She also enjoys the backing of 40% of Labour and Liberal Democrat supporters and an impressive 48% of Conservative voters.
London’s electoral system means any mayoral candidate needs to win 50.01% of the votes cast, making the backing of other parties’ voters essential to victory.
Jowell’s closest rival is veteran MP Diane Abbott who is backed by 18% of all Londoners and retakes second place after last month being pipped by Sadiq Khan.
With the general election campaign now underway Mr Khan has been enjoying increased levels of media coverage yet his ratings have slipped 2 points this month to 14%, the same level enjoyed by rival David Lammy.
Behind the pair are Doreen Lawrence on 10% and transport expert Christian Wolmar on 3%.
Labour will select their candidate in an all-London primary in which non-members will also be allowed to vote in return for paying a small registration fee.
