Labour's backing in London has fallen by nine points over the past year, while Liberal Democrats have doubled their support in the capital since the EU referendum, according to a new poll.
The YouGov survey for Queen Mary University of London put Labour on 37%, just three points ahead of the Conservatives on 34%, with Lib Dems on 14% and Ukip down four points on 9%.
London is a key battleground for Labour, which took 44% of the capital's vote in the 2015 general election and holds 45 seats in the city, including Jeremy Corbyn's Islington North constituency, John McDonnell's Hayes and Harlington and Diane Abbott's Hackney North & Stoke Newington.
The party has also held City Hall since May 2016, when Sadiq Khan was elected mayor.
Mr Khan recorded a strong approval rating of plus-35 in the poll, with 58% of those surveyed saying he is doing a good job.
But fewer than one in five voters in the capital thought Mr Corbyn was doing a good job, while 62% thought he was doing badly, giving him an overall raging of minus-44.
Mr Corbyn's score was negative among every political or demographic group in the capital, including age, gender, social class, remainer/leaver, inner/outer London, and party support. Even among those who voted Labour in 2015, he had a rating of minus-35.
The Lib Dem resurgence may reflect London voters' strong support for remaining in the European Union, which was a key factor in Sarah Olney's victory in the Richmond Park by-election in December, which brought the party's representation in the capital up to two.
The poll recorded broad cross-party approval of Mr Khan, who has support from the majority of Labour voters as well as a majority of Lib Dems.
Even among Conservatives more think he is doing well than think he is doing badly.
His predecessor as mayor, Boris Johnson, scored minus-14 among Londoners for his performance as Foreign Secretary.
QMUL professor Philip Cowley, the director of the Mile End Institute, said: "When people are asked why they approve of Khan, the responses rarely include specific policies.
"Supportive responses instead focus on his lack of any errors to date, the fact that he isn't Boris, the belief that it is too early to tell along with a broader sense that he is saying and at least trying to do the right things for Londoners. Overall, Londoners like him."
Some 46% of those questioned said Theresa May was doing well as Prime Minister, giving her a net favourability rating of plus nine.
:: YouGov questioned 1,042 adults online between March 24 and 28.