Labour’s deputy leader, Tom Watson, has said the decision to expel Alastair Campbell for voting Lib Dem was spiteful and members who voted for other parties in the European elections should be given an amnesty.

In a rebuke to Labour’s HQ, Watson said members who voted for other parties should be listened to rather than punished. Campbell, Tony Blair’s former spin doctor and a leading advocate for a people’s vote, admitted after polls had closed that he had voted Liberal Democrat.

The expulsion of Campbell led to a torrent of admissions from senior Labour figures that they had also voted for other parties in protest at the party’s position on Brexit and the hashtag #ExpelMeToo was briefly trending on Twitter.

The former Labour home secretary and party chairman Charles Clarke said he had also voted Liberal Democrat in the election, while the ex-defence secretary Bob Ainsworth said he voted Green.

Fiona Mactaggart, a former Labour minister, admitted she also voted Lib Dem and said it was “time for us all to declare: ‘I am Spartacus.’”

Watson said members who had voted elsewhere to send a message to Labour had been right about the party’s lack of clarity on the issue of a second referendum, compared with parties such as the Greens, Lib Dems and the SNP.

“It is very clear that many thousands of Labour party members voted for other parties last week,” he said. “They were disappointed with the position on Brexit that a small number of people on the NEC inserted into our manifesto. They were sending the NEC a message that our position lacked clarity, and they were right.

“It is spiteful to resort to expulsions when the NEC should be listening to members. The politics of intolerance holds no future for the Labour party. A broad church party requires pluralism and tolerance to survive. There should be an amnesty for members who voted a different way last week. We should be listening to members rather than punishing them.”

On Tuesday night Campbell said members of Jeremy Corbyn’s staff were among those who sent messages of support for his decision. He said he would appeal against the expulsion and warned Labour faced electoral oblivion unless it clarified its position.

He tweeted: “Among many, many messages of support have been some from Corbyn’s office, shadow cabinet, MPs, union leaders and party staff including one telling me there is a mountain of emails from members who responded to mobilisation email by saying no to campaigning and not voting Labour.”

In a statement of support for Campbell, Clarke said he had been a member for 47 years but had also voted Lib Dem. “I was not aware that Alastair had voted Liberal Democrat in the European election until I heard him say so on television on Sunday evening. His expulsion from Labour party membership is a disgrace and only compounds Labour’s current political difficulties.”

The former lord chancellor and Labour peer Lord Falconer said it remained unclear whether the rules had been followed correctly.

“I don’t think they have. The rules say you’re not allowed to support another political party; does voting for another political party and only saying you’ve done it after the event involve ‘supporting another political party’?” he told BBC Radio 4’s Today programme.

“If it was an offence under the rules, then I suspect thousands, maybe tens of thousands, of members of the Labour party at the European elections broke the rules and that’s not a tenable position.”

Falconer, who said he had voted Labour and did not back a second referendum, said the decision to swiftly expel Campbell was “bound to have been taken high up the chain” and said it would send a message about what the disciplinary process would prioritise.