Almost one third of voters are not planning on backing any of the three main parties, according to research.
A ComRes poll for the Independent put support for "others" at 30%, up 18% since before the expenses row erupted.
The rating is neck and neck with the Tories - who have plummeted 15% from the equivalent poll last month.
Labour has dropped four points to 22%, while the Liberal Democrats were up one on 18%. The Greens were on 8%, Ukip on 7%, the BNP 3%, the Scottish National Party 2%, Plaid Cymru 1% and other smaller parties or independents 9%.
Meanwhile, an Ipsos Mori poll found support for Labour has plunged by 10 points in the past month, and the party is now running neck and neck with the Liberal Democrats.
The dramatic slump in Labour's fortunes has allowed the Conservatives to pull 22 points clear, according to the research.
While support for all the main parties has declined in the aftermath of the MPs' expenses scandal, Labour appears to have fared the worst by far.
The Ipsos Mori poll put the Tories on 40%, down one since the same survey last month. But Labour was down 10 points, putting the party on the same support as the Lib Dems, who were down four on 18%.
At the same time, support for minor parties has doubled in the past month to 24%, including 7% for the UK Independence Party, 6% for the Greens and 4% for the British National Party.
The poll will harden expectations that Labour will be given a drubbing in Thursday's European and local elections.
The big problem seems to me that the smaller parties could cancel each other out, especially in the Euros, thus allowing the big three in.
In my county council division the smaller parties are either not standing or have merely put up a paper candidate. They just don't get it, they will not win parliamentary seats until they prove what they can do at council level.
So for now, it looks like we are stuck with big empty gestures and useless protest votes, only the Green Party, of the smaller ones, actually seem to be taking politics seriously.