Senior Labour figures have warned that the party could lose safe seats to Ukip in the wake of an unexpectedly narrow victory in the north of England.
Fears that Ukip could threaten Labour as well as the Conservatives at next year's General Election threaten to re-open doubts within the party about Ed Miliband's leadership.
Ukip came within just over 600 votes of Labour in the Heywood and Middleton constituency, in Greater Manchester.
The area had been seen as a safe Labour seat.
Mr Miliband acknowledged his party needed to "reach out" to disaffected voters after seeing one of his northern strongholds come close to falling.
But he faced warnings that Labour risked doing too little to address their concerns.
At the same time he faced criticism that he was concentrating too much on core Labour voters - many of whom backed Ukip - instead of broadening the party's appeal.
Speaking in the constituency, Mr Miliband said Labour would do more to listen to the concerns of local voters. He said the party needed to demonstrate it could "listen and deliver" on jobs , housing and wages.
"These results show the fight that we face over the coming seven months" he said. "This is a fight against disillusionment and despair, this is a fight for the working people of Britain and this is a fight I am determined to win."
While his message also appeared to be an appeal for calm among Labour MPs as much as anything else, one, John Mann predicted that over the next 48 hours his party's leadership would change tack.
Mr Mann said: "If Ed Miliband does not broaden the Labour coalition to better include working-class opinion, then we cannot win a majority government. Ed Miliband does a lot of listening. Now he needs to do a bit more hearing."
Former Labour minister Frank Field said: "If last night's (Thursday's) vote heralds the start of Ukip's serious assault into Labour's neglected core vote, all bets are off for safer, let alone marginal, seats at the next election."
Ukip leader Nigel Farage said the result showed how successfully his party could take votes from Labour as well as the Tories.
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