Results for local council ballots will be keenly studied in Westminster for pointers towards the 2015 general election.
Political analysts say a surge in support for the UK Independence Party (UKIP) has left all three main parties nervous, with the Conservatives and Liberal Democrats particularly expecting to suffer at the hands of Nigel Farage's candidates.
Labour was confident of success in the South Shields by-election prompted by David Miliband's decision to quit UK politics, despite a strong showing by UKIP.
A senior Labour source said the party was "quietly confident" of retaining South Shields, which it won in 2010 with 52 per cent of the vote.
Bookmakers are predicting Mr Farage's candidates could scoop more than 100 seats, and Prime Minister David Cameron acknowledged that Conservatives face losing votes to UKIP.