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Labour leader Sir Keir Starmer has cast his vote at Greenwood Centre polling station at St Albans Church, north London.
Ballots are being cast across Great Britain in the largest test of political opinion outside of a general election.
Sir Keir is under pressure as the future of the Labour Party is among the issues at play.
The Hartlepool parliamentary by-election will indicate what progress - if any - Labour has made in regaining votes in its former northern heartlands.
Defeat would be a blow to Sir Keir and provide a rare by-election gain for a governing party.
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