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.

British Prime Minister Keir Starmer marks a year in office this week, facing challenges from within his party and the public.

UK Prime Minister Keir Starmer avoided a humiliating parliamentary defeat on key welfare reforms on Tuesday, after making last-minute concessions in the face of the most serious internal rebellion of his year-long premiership.