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Nigel Farage’s Ukip is rising in popularity in the run-up to the party conference season and the Clacton by-election, an exclusive poll reveals today.

Ukip has jumped two percentage points in the past month to stand at 15 per cent, its equal-highest level, while the bigger parties have failed to make significant headway, Ipsos MORI found.

The monthly poll, published first in the Evening Standard, destroys David Cameron’s hopes that the anti-EU party would dwindle in the polls after its victory in the local and European elections in June.

In other findings that will widen the smile on Mr Farage’s face:

More than half, some 53 per cent, say Ukip is “highlighting important issues which other parties aren’t taking seriously enough”.

The proportion of people who think voting Ukip in a general election means “a wasted vote” has declined. The percentage who agree has fallen from 57 to 50 since May, and the percentage who disagree has risen from 33 to 41.

Mr Farage has by far the least toxic net satisfaction ratings out of the four biggest party leaders.

The Conservatives are up a point to 34 per cent, with Ed Miliband’s Labour unchanged on 33 per cent.

It is the first time since December 2011 that the Tories have crept ahead of Labour — but Ipsos MORI experts say the lead is so slender before statistical rounding that the parties should be seen as running neck-and-neck.

Nick Clegg’s Liberal Democrats are marooned on seven per cent for the second month running, a fraction of the 23 per cent they polled at the 2010 general election. Moreover, Mr Clegg’s personal ratings have slumped this month to their equal worst yet, with 66 per cent dissatisfied with his performance and 21 per cent satisfied.

The poll suggests the conditions could be ripe for a Ukip triumph at Clacton on October 9.

Meanwhile a buzz is sweeping Westminster that the party is gaining traction for a second by-election being held on the same day, at Heywood and Middleton where the late Labour MP Jim Dobbin had a majority of 6,000.

However, only 14 per cent say they would like their MP to copy Tory defector Douglas Carswell and join Ukip. Three quarters would oppose such a defection.

Gideon Skinner, head of political research at Ipsos MORI said: “Only 74 per cent of voters with a preference choose one of the three main parties — history tells us that’s unusually low, and a fall from nine in 10 even in 2010.

“In other words, one in four are giving their vote to a party outside the main three, highlighting the troubles they all face — and Ukip are the main beneficiaries.”

A striking finding is that more Labour Party supporters say they are dissatisfied with Mr Miliband (46 per cent) than the 42 per cent who say they are satisfied with his performance as Labour leader.