By Laura Kuenssberg
Political correspondent, BBC News
The poll suggests UKIP has seen a particular surge in support
The traditional assumption that support for political parties other than the main three disappears at a general election has been challenged by private polling seen by the BBC.
And just ahead of their party conference, the research suggests that support for the Conservatives in some of their target seats has fallen slightly in the last year.
The number of voters in marginal seats who said they would vote Labour instead has also stalled.
But the striking change suggested by the data is the rise in the number of people who told pollsters they would vote for parties other than the big three.
The polling was carried out by one time Conservative strategist Lynton Crosby, who spearheaded Michael Howard's general election campaign in 2005.
It suggests that 44% of voters in key marginal seats like Crawley and Hove in the south of England would vote Tory at a general election - down from 48% last year.
Support for Labour is languishing at 20%, while the Lib Dems are down from 21% to 18%. But support for "others" - UKIP, the Green Party, the BNP and other small parties - has doubled, from 9% to 18%.
The polling suggests a surge in support for UKIP, in particular.
In summer 2008, just 1% of people in marginal seats said they would back the party at a general election. Now that figure has risen to 6%.
The Greens have seen a smaller increase, from 4% to 6% while the BNP rating has gone up by just one point, from 2% to 3%.
The increase in support for the lesser-known parties has come at a cost to all of the major ones, the research suggests.
But the Tories may be most worried about the downward shift as it suggests they have been just as badly affected by the public's anger over the expenses scandal as Labour - if not more so.
The research, which was carried out in July for the pressure group Flying Matters, also reveals that the drop in Labour's support matches almost directly the switch of the party's leader from Blair to Brown.
But it also suggests that about a quarter of people's allegiances are "soft", meaning they are yet to make up their minds.
Worries about jobs, not surprisingly, have gone up the list of voters' concerns. Healthcare and crime were the second and third most important priorities, while concerns about the environment have become less of an issue in voters' minds.
It is, of course, only one piece of polling, and it would be dangerous to translate it into solid conclusions.
But it does give a rare glimpse of political shifts in the places around the country where the next general election will be decided.