Tory lead rises to nine points in latest ICM poll and eleven points in Opinium survey.
Poll suggests an increasing number of voters would like to see a hung parliament.
Latest YouGov poll would leave Brown 19 seats short of a majority.
David Cameron and his party look short of the seats they need for a majority.
Latest daily YouGov poll suggests Labour could emerge as the single largest party.
New poll suggests David Cameron will struggle to win an overall majority.
Party reaches crucial figure of 40 per cent for first time since mid-February.
Voters also expect David Cameron to perform best in the leaders' debates.
Latest figures would leave Labour as the largest single party.
Tory lead recovers but hung parliament is still likely.
Tory lead cut to just two points in new YouGov poll.
Latest Ipsos MORI poll puts Tory lead at lowest level since December 2008.
YouGov poll for the Sunday Times (21 February) reveals narrowest Tory lead since 2008.
Conservatives on track for a small majority as party rises two points.
Tory lead rises to 12 per cent in first poll since leadership crisis
Back in hung parliament territory
New survey shows Tory lead narrowing to 10 points
Poll finds 57 per cent oppose independence as Salmond prepares to publish referendum plans
Party boosted by public sympathy for Brown over Sun attacks
Conservatives lead Labour by 10 points but would win a majority of only two
Figures would give Conservatives a Commons majority of 106
Party on track for a majority of 58 at the next election as voters back Cameron's anti-statism
Conservative lead cut to 11 per cent as Labour rises three points
Party boosted by the conference season but support for Cameron's leadership rises
Tories also lead Labour on every policy issue except the NHS
First poll since David Cameron's conference speech shows party's lead rising by 8 per cent
Party halves the Tory lead to seven points on the back of its conference
Poll also shows that a majority of voters remain unsure that there is "substance" behind David Camer
Party on track for a majority of 96 seats at the next election