This article is more than 7 years old

This article is more than 7 years old

One in five voters support Ukip according to a new poll providing evidence of a continued surge in support for Nigel Farage's party.

The anti-European party has reached 21% in the polls in a new high. Labour are unchanged from the last Observer/Opinium poll a fortnight ago, on 37%, while the Conservatives drop to 26%.

Nick Clegg's Liberal Democrats has sunk to 6% – the lowest proportion of the vote recorded by Opinium for the party.

Meanwhile David Cameron's approval rating continues to tick downwards, with 28% approving compared to 29% who don't in the last poll.

The poll will be a blow to the Tories, with the position of one of its MPs, Patrick Mercer, looking in jeopardy following claims that he has tabled parliamentary motions after taking thousands of pounds from a fake lobbying firm in a newspaper sting.

Mercer's constituency, Newark, is regarded as a Ukip stronghold. Farage was recently invited to talk to Mercer's constituency by the MP. Farage told the Observer that he came away with the impression that the Tory constituency association was "closer to us than to their own leader". Farage denied that Mercer had any plans to switch to Ukip, claiming the former army officer had a strict code of loyalty.

Mercer has resigned the Tory whip following the claims by the BBC's Panorama and the Daily Telegraph. He is now coming under pressure to resign from the Commons before the next general election.

Tory MP Zac Goldsmith tweeted: "If it's bad enough for you to resign from your party, how can it be ok to continue representing constituents at all? Where's that Recall?!"

Farage said he would not stand in Newark if Mercer did quit but another Ukip candidate would be found.

• Opinium Research carried out an online survey of 1,948 GB adults aged 18+ from 28 to 31 May 2013. Results have been weighted to nationally representative criteria. Full tables and results can be found here