Shadow justice minister Sadiq Khan has said Ed Miliband's route to Downing Street is through electoral success in London.
Mr Khan told ITV News that Labour's win in David Cameron's "favourite council", Hammersmith and Fulham, showed that the party's message was resonating with voters in the capital.
UKIP's successes in the local elections have raised the possibility of them winning a Westminster seat at the general election.
A year ahead of the general election, Ed Miliband's party is seeing only patchy support in areas it should expect to gain ground.
UKIP are likely to make Rotherham a key target for 2015 after a storming result in the local elections.