Labour is now certain to take overall control of the London borough of Merton - the third council it has gained so far after Hammersmith and Cambridge.
After 48 of 60 seats had been declared, the party, which has ruled as a minority administration, had won 33.
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.