The bitter fallout from Brexit is threatening to break the Tory party apart, as a Europhile former cabinet minister Stephen Dorrell on Sunday announces he is defecting to the independent MPs’ group Change UK, and a new opinion poll shows Conservative support plummeting to a five-year low as anti-EU parties surge.

Writing in Sunday’s Observer, Dorrell, who was health secretary under John Major, says he can no longer continue in a party that “has fallen progressively under the influence of an English nationalist outlook” and turned its back on the traditions of many of its greatest former leaders.

Arguing that neither the Conservatives nor Labour now represent mainstream opinion in the UK, Dorrell says that the current two-party system no longer serves the interests of the electorate. He writes: “I shall continue to describe myself, as I always have, as a liberal Conservative but I shall do so in future as a supporter of Change UK – The Independent Group, which I believe has become the natural home of those who regard themselves, as I do, as the heirs of Disraeli, Churchill, Macmillan and Heath.”

He adds: “I do not believe any of them would have countenanced the idea that Britain should imagine its interests are best served by removing itself from the European mainstream.”

Facebook Twitter Pinterest Stephen Dorrell said the Tories had ‘fallen progressively under the influence of an English nationalist outlook’. Photograph: Graeme Robertson/The Guardian

Dorrell is understood to be interested in standing in next month’s European elections for Change UK, if they go ahead. Another former Tory MP, Neil Carmichael, who was chairman of the Commons education select committee, has also left the party and wants to stand for Change UK in the European elections.

Carmichael said he could not carry on as a Conservative. “I think the attitude of the Conservative party at the moment is simply wrong. It isn’t easy for the prime minister – she was dealt a bad hand. But the ‘Brexit means Brexit’ slogan got us off to a hard Brexit start and her red lines on leaving the customs union and single market did not help thereafter.

“I joined the party in 1989. You reach a point where you have to stand up and say, this is just not right. I’m hoping the electorate will deliver a message to the party. I’m thinking about standing in the European elections and would do so for the Independent Group. That is my intention. It is an opportunity to redefine the debate and excite people over the prospect of a people’s vote.”

In a further sign that the Conservatives would face a split if May were replaced as leader by a hardline pro-Brexit leader, the Observer has also obtained a recording of Kenneth Clarke, the former chancellor, telling students at Nottingham University earlier this month that, while he believes that leaving the Tory party now would be an “odd gesture”, he would “probably” switch to Change UK if May’s successor was a hardline Brexiter leader who appointed a cabinet full of anti-EU Tories.

Play Video 0:41 Ken Clarke says he 'would probably' join TIG if hard Brexiter becomes Conservative leader – video

In February eight Labour MPs and three Tories quit their parties to form Change UK – The Independent Group, which has applied to the Electoral Commission to be able to enter candidates in the European elections on 23 May. The elections will take place if no Brexit deal has been passed by MPs by 22 May, under a proportional representation list system. Tom Levitt, a former Labour MP, has also quit Labour to join the fledging new party and he too hopes stand for Change UK at the European elections.

Facebook Twitter Pinterest Kenneth Clarke. Photograph: David Levene/The Guardian

The latest defections come as a new Opinium poll for the Observer shows a dramatic fall in Tory support in the past two weeks and a surge for anti-EU parties. The Conservatives have fallen by six percentage points to 29% compared to a fortnight ago. It is their worst position since December 2014. Labour is up one point on 36% while Ukip is up two points on 11%.

Even more alarmingly for the Tories, their prospects for the European elections appear dire. Only 17% of those certain to vote said they would choose the Conservatives in the European poll, while 29% would back Labour, and 25% either Ukip (13%) or Nigel Farage’s new Brexit party (12%).

Adam Drummond of Opinium said last week’s decision by EU leaders to allow an extension to Brexit had hit Tory support. “Extending article 50 until October has done predictable damage to the Tories’ public support with the largest lead for Labour since 2014. The government also looks set to take a pasting in the European parliament elections next month with the Leave vote fracturing between it, Ukip and Nigel Farage’s Brexit party, while much of how the Remain vote splits will depend on how well Labour can continue straddling the divide. However, it’s important to emphasise that if these elections take place, they will be in totally unprecedented circumstances and therefore the scope for dramatic changes is huge.”

Separately, polling expert and Conservative peer Robert Hayward said the Tories were likely to lose more seats than other parties in the local council elections on 3 May, partly because they had performed strongly when the same seats were last up for election four years ago and the Liberal Democrat vote collapsed. Hayward said the Lib Dems were likely to be the biggest gainers, while Labour, which had fared poorly in recent council byelections, might do less well than its supporters hoped.