Another senior Conservative peer has been suspended from the party whip for pledging to vote Liberal Democrat in the European elections, after the punishment was imposed on the former deputy prime minister Michael Heseltine.

Andrew Cooper, the founder of pollster Populus who was David Cameron’s director of strategy in Downing Street, tweeted: “I have come to the same conclusion as Michael Heseltine, for exactly the same reasons – and will be voting Lib Dem in Thursday’s European parliament elections.”

He was subsequently told by the Tory party’s Lords’ chief whip, John Taylor, that “endorsing the candidates of another party is not compatible with taking the Conservative whip”.

It comes after Gus O’Donnell, the former civil service chief, abandoned his political neutrality to say he would vote for the Liberal Democrats.

The ex-cabinet secretary, nicknamed God, wrote in the Times that the clearest choice for a remain party was “voting Liberal Democrat in England, so that’s what I will do”.

He said: “I would urge all those who support remain to do the same. It feels very strange to be specifying a preference for a particular party.

“However, as a crossbencher in the Lords, and faced with a decision that will affect generations to come, I believe it is my civic duty to vote and there is now no reason not to be clear about how I use this precious power that democracies bestow on their citizens.”

The Labour peer Michael Cashman, a former EastEnders actor, said: “I can’t trust [Jeremy] Corbyn or the people around him on the defining issue in postwar Britain, so on Thursday I will not be voting for the Labour party. As Matthew Parris said, I am not a Liberal Democrat, but I support their absolute consistency. Voting Lib Dem in the EU elections.”

He added: “I think I’ve just resigned from the Labour party by declaring that I will support the Liberal Democrats in the European elections.”

Parris, the former Tory MP and newspaper columnist, had earlier added to those from other parties saying they would lend their vote to the Liberal Democrats, telling the BBC’s Newsnight: I’m not a Liberal Democrat, I belong to the Conservatives … but they are the only party in this election … that believes we shouldn’t leave without a referendum.”

He said he would recommend voting Liberal Democrat to any Tory wishing to remain in the EU.

Heseltine, the veteran Tory, had the whip suspended for saying he would vote Liberal Democrat at the European elections.

The declarations mark the latest sign that the Liberal Democrats are becoming the pre-eminent party of remain in England, followed by the Greens and Change UK. There has been frustration among remain voters about the lack of an alliance among those parties. Of the three, the Liberal Democrats have consistently polled the highest.

Meanwhile, Nigel Farage, the Brexit party leader, and Vince Cable, the Liberal Democrat leader, took part in a head-to-head debate on Wednesday in a sign the European elections are becoming a proxy second referendum, with voters polarised along remain and leave party lines.

Farage accused Cable of not having “democracy running deep in your bones” because of his support for a second referendum. He predicted a big win for the Brexit party would “kill off another chance of a second referendum” and spell the end of May’s premiership.

Cable made the case for stopping Brexit and staying in the EU to deal with the climate emergency. “It’s not simply the damage caused by Brexit but the positives of being part of the EU,” he said.