By Andrew MacAskill, Elizabeth Piper and Alistair Smout LONDON (Reuters) -British Prime Minister Keir Starmer won a vote on his welfare plans on Tuesday at significant political cost as he suffered the biggest parliamentary rebellion of his premiership and was forced to back down on key parts of the package.

Keir Starmer is facing the largest Labour rebellion of his time as prime minister as he tries to force through controversial welfare cuts. Dozens of his own backbenchers are set to defy him to vote against the Universal Credit and Personal Independence Payments Bill in the Commons on Tuesday evening.