Observer/Opinium poll shows 31% of voters would back Nigel Farage’s party if they believed it could win in their constituency

The phenomenal rise in support for Ukip is underlined by a new Opinium/Observer poll which shows almost one-third of voters would be prepared to back Nigel Farage’s party if they believed it could win in their own constituency.

While the survey, which puts the Conservatives and Labour neck-and-neck on 33%, shows a substantial boost for the Tories (up five points on a fortnight ago), the rise of Ukip will be deeply alarming to the main parties.

With just over three weeks to go before a crucial byelection in the normally safe Tory seat of Rochester and Strood, which Ukip threatens to seize, the poll puts Ukip on 18% of the national vote, with the Lib Dems on 6% and the Greens on 4%.

If the Ukip candidate Mark Reckless, who defected from the Tories last month, wins the byelection, the Conservatives fear there could be a rush of defections as MPs conclude that their chances of re-election are higher under Ukip colours.

When asked to respond to the statement “I would vote for Ukip if I thought they could win in the constituency I live in”, 31% of voters said they agreed. This includes 33% of Tory voters, 25% of Liberal Democrats and 18% of Labour supporters. Voters were equally divided on whether a vote for Ukip was a wasted one, with 40% saying it was, and 37% saying it was not.

The poll also shows all four main party leaders on negative net ratings, with Nigel Farage the most popular on just -1%, David Cameron on -6%, Ed Milband on -23% and Nick Clegg on -43%.

On Saturday night, Robert Ford, who co-authored a recent academic analysis of Ukip support with Matthew Goodwin, Revolt on the Right, said the findings raised new questions about next year’s general election, and suggest Ukip could be even more of a threat in subsequent ones.

“These figures could have serious implications for all the main parties. In seats where Ukip are already well-established thanks to local election and byelection success, there will be a large pool of voters they can appeal to in their attempts to build a winning Westminster coalition in 2015.”

The biggest effect of the Ukip surge could be in subsequent national elections. “In many seats, Ukip support may currently be suppressed by the perception that they are a ‘wasted vote’ – a perception which a party with little electoral track record will find it hard to combat,” he said.

“Yet if Ukip perform in line with current polling, they will secure strong second-place finishes in a wide range of seats next year, and then, like the Liberal Democrats before them, they can take their case to voters as the party of local opposition. The large swath of the electorate willing to seriously consider the party will make this a viable option in a wide range of seats, potentially opening a wide swath of constituencies to an unprecedented challenge.”

The latest sign of the rise of Ukip – which campaigns for the UK to leave the EU – follows an unexpected attempt by the European commission to get the UK to pay £1.7bn more into European coffers following a budgetary readjustment, based on the gross national incomes of member countries.

The commission said the budget was always adjusted to take account of countries’ gross national incomes but provoked a furious response from David Cameron, who said he would block the payment by the due date of 1 December. The prime minister will make a Commons statement on Monday spelling out his intentions.

Farage has said Cameron will have no option but to pay up and cited the row as further evidence that the UK should leave the EU as soon as possible.

In a sign Tory MPs may call on Cameron to block any extra payment, Nick de Bois, Enfield North MP and secretary of the backbench 1922 committee, said he believed the UK should stand firm and pay nothing extra.

He said: “The prime minister is absolutely right to set out that he won’t pay it – and I have every confidence that is exactly what will happen.”

Chris Leslie, the shadow chief secretary to the Treasury, wrote to George Osborne demanding to know how long the government had known about the possibility of a higher EU budget surcharge. Cameron said he only found out earlier this week, but the Treasury had known of the demand for some days before that.

Leslie said in his letter there was evidence that the Office for National Statistics, which approved the data used for the calculation, had known for some time about the pending charge. “The ONS published a report over four months ago detailing the changes made to UK growth figures which it stated were for use in the calculation of a member state’s contribution to the EU budget.

“These surcharges are the conclusion of a process launched in 2011 by the European statistics agency Eurostat, and cited by the ONS in 2012, which was designed to harmonise the GNI calculations for EU nations.”