Support for the Labour Party in Britain is at its lowest level in a nationwide election since 1910.
Early results from elections for the European Parliament held last Thursday are being announced on Monday.
A projection for the BBC gives Labour 16% of the vote, just behind the anti-EU UK Independence Party and 11 points behind the Conservatives.
Labour's share of the vote was down about seven points from the last European election in 2004, while the Conservatives were little changed.
The dismal results have prompted fresh calls for Prime Minister Gordon Brown to step down.
Labour has been hurt by a scandal over expense claims by Members of Parliament and questions over Mr Brown's leadership at a time of deep recession. Six senior cabinet ministers quit the government last week.
Mr Brown reshuffled the cabinet on Friday after the resignations, and told supporters on Sunday that he would not walk away from the country's troubles.
However, rebel MPs are due to meet on Monday and could go public with their criticisms of the leader.
Win for far-Right BNP
The far-right British National Party has won its first seats in the European Parliament. The gains were made at the expense of Labour.
The party won a seat each in the northwest of England and in Yorkshire and the Humber and is projected to win 6.5% of the vote nationally.
It is a breakthrough for the anti-immigration party, which until now has had no seats in the British or European parliaments.
Party leader Nick Griffin, who was elected, said it was not racist but spoke for "ordinary people" neglected by other parties.
Labour also lost a seat in Wales, a traditional stronghold.