Doubts about Ed Miliband's leadership are said to be driving an exodus in the runup to next year's general election

More than 15% of Ed Miliband's MPs are set to quit parliament before the next election, raising fears among senior Labour figures of a "brain drain". Thirty Labour MPs – 15 with ministerial experience – have announced that they are leaving, despite polls suggesting that the party will form the next government.

A senior Labour party source said it expected at least 10 more of its MPs to stand down before the election, pushing the proportion of those leaving to 15.5% of the parliamentary party. Before Labour came to power in 1997 under Tony Blair, only 8% of the party's MPs quit. A source said: "There's not enough excitement about Ed. Some worry about him losing the general election, and some worry about him winning it."

The last few months have seen a welter of big hitters announce their retirement, including former home secretary David Blunkett and former health secretary Frank Dobson.

Former chancellor Alistair Darling, who is leading the campaign against independence for Scotland, and the shadow international development secretary Jim Murphy are rumoured to be considering their positions. Darling has said he will decide "in the days following the referendum if I stand again".

Murphy declined to comment on rumours that he was considering a move to Scotland to enter the Holyrood parliament at the 2016 election. Murphy, who ran David Miliband's leadership campaign, has been sidelined in recent years by the Labour leader, who moved him from defence to shadow Scotland secretary and, most recently, to international development.

Any move by Murphy would be motivated by the chance of entering a Scottish parliament with new powers, following the announcement by all three party leaders that such reform would happen if Scotland rejected independence. However, other Labour MPs are reacting against what they see as the uninspiring character of their leader.

The removal of veteran Labour MP Dennis Skinner from the party's governing national executive committee is being blamed on people in the leader's office who encouraged the 2010 intake of MPs to support John Healey, a former shadow health secretary, in his bid for a seat. Critics believe that Healey was bolstered in his bid to damage the chances of Liverpool Walton MP Steve Rotheram, who was critical of Miliband for posing with a copy of the Sun.

A source said: "It looked like an attempt to show the strength of the leader. But Rotheram is a good campaigner, so he got his seat. Healey got his seat. But poor Dennis, who they thought was a shoo-in, didn't campaign and didn't get the votes. Miliband's people couldn't even get that right."

When the vote was announced, Murphy tweeted: "Really sorry to hear that Dennis Skinner was voted off Labour's NEC today. A brilliant MP & good friend. Hopefully he'll make NEC comeback."

About 11% of Tory MPs have already announced that they are standing down with some reports suggesting that a quarter of the party's female MPs will have announced their intention to leave parliament before 2015.