The Clacton poll put Ukip on 64 per cent, the Tories on 20, Labour on 13 and the Lib Dems on 2. More than half favoured quitting the European Union, with 54 per cent supporting a British exit and 26 per cent opposed to cutting ties to Brussels.
Fifty seven per cent said it was because they liked Ukip and nine per cent said it would be a protest vote.
"If we lose, then selling ourselves as a party that can win a significant number of seats will get a lot harder.
"If Douglas wins this by-election, then there will be more and more people coming over to us. We could end up holding the balance of power by next May."
He added: "I'm going to keep playing double or quits until I lose.
"But if I win three more hands, we'll be out of the EU and a free country again. There is a massive appetite out there for change." Mr Farage's party has made inroads in Labour's northern strongholds in recent elections, and he said: "We're going all out for the workingclass vote now. Five of our most winnable seats are Labour-held and there are even some Labour MPs thinking of joining us."
Mr Goldsmith took to Twitter yesterday to dispel newspaper suggestions he was going to defect if the Government changes its stance on green subjects.
"I was critiquing Ukip's position on green issues to illustrate why I couldn't join them," he said.
Tory MP Nigel Evans said: "I fought the Ribble Valley by-election in '91. It was the 13th safest seat in the land and we lost it and a year later I was the MP. So how important are these by-elections?"