Votes are being counted in three parliamentary by-elections after polls closed.
Voters turned out in Croydon North and Middlesbrough to elect new MPs following the deaths of Malcolm Wicks and Sir Stuart Bell. Rotherham will also get a new MP after Denis MacShane's resignation in the wake of a damning report condemning him for abusing parliamentary expenses.
Labour will hope to retain all of the Commons seats despite signs of a strong showing for smaller parties.
A senior source said the party was quietly confident in Middlesbrough and Croydon North, where around one fifth of voters were predicted to have turned out.
But the it is less confident in Rotherham, despite claiming the Labour vote has held up, because it is not sure where other votes going
Its is odds-on favourites with bookmakers, however, to keep hold of all three - won with healthy majorities in 2010 - with Middlesbrough seen as a near-certainty.
But there will be close attention on whether Respect can pull off a repeat of George Galloway's shock April victory in Bradford West in either of the others.
Mr Galloway overturned a 5,000 Labour majority - smaller than any being defended by Labour today - to storm home with a 10,000-vote lead.
Another threat is posed by the UK Independence Party, which pushed the Liberal Democrats into third place in the Corby by-election a fortnight ago.
Its profile has been significantly boosted in Rotherham by the storm over the removal by the Labour-run council of foster children from a couple because of their Ukip membership.