Sources say Ed Miliband is expected to stand down after Labour was all but wiped out by the SNP in Scotland.
Lib Dem leader Nick Clegg is also expected to quit with his party set to be reduced from 57 to eight MPs.
There is also speculation about the future of UKIP leader Nigel Farage, after he failed to win Thanet South.
Mr Farage, who lost to the Conservatives by nearly 2,800 votes in the Kent seat, has said he would step down if he did not make it to Westminster.
The BBC forecast, with 635 of 650 seats declared, is Conservative 331, Labour 232, the Lib Dems eight, the SNP 56, Plaid Cymru three, UKIP one, the Greens one and others 19.
The Conservatives are expected to have won a 37% share of the national vote, Labour 31%, UKIP 13%, the Lib Dems 8%, the SNP 5%, the Green Party 4% and Plaid Cymru 1%.
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