Former Ukip leader Paul Nuttall appeared on ITV’s flagship election debate without reading his party’s manifesto, Ukip’s general secretary has disclosed.

Jonathan Arnott, the party’s constitutional affairs spokesman, said the “bland, insipid” policy plan was drawn up by Nuttall’s advisers but not shown to him in full before the ITV Leaders’ Debate on 18 May.

He told the Guardian: “The 2017 manifesto was certainly well costed, but it lacked something: it didn’t really say very much of any consequence. It was a bland, insipid document with large chunks of policy simply missing from last time.

“Not that the manifesto was Paul Nuttall’s fault, of course: he went into the ITV debate with nobody having even done him the courtesy of showing him the entire manifesto draft. Paul found himself regurgitating visions of cutting class sizes in primary schools, blissfully unaware that the policy had – like so much else – been quietly binned.”

Nuttall quit as Ukip leader on Friday morning after the party’s disastrous election results. Ukip won 1.8% of the vote on Thursday, a dramatic fall from its 12.6% share in 2015. Nigel Farage told a Sunday newspaper he was “50-50” about making a fresh leadership bid and would decide in the coming days.

Arnott, Ukip’s MEP for north-east England, said the party was in a “last-chance saloon” and must ditch its “hardline anti-Islam messages” if it was to return from electoral oblivion.

He said on Friday that the manifesto had been shown to Ukip’s NEC “at the very last minute, who had barely an hour to consider it and were therefore bounced into voting to approve a document that simply wasn’t good enough”.

On Sunday, Arnott described the party’s burqa ban as “an odd choice for a first big speech of a campaign (and personally I disagree with such a blanket ban)” and said Nuttall’s closest advisers “pushed him into it”.

Ukip’s proposal for school-based examinations of potential victims of female genital mutilation was “not only divisive and wrong, but also counterproductive in that the policy detracted from a very serious message”, he said.

Arnott added: “I’ve watched in disappointment, bordering on despair, as I’ve seen candidates and councillors fail to recognise the difference between Muslims as a whole, and the tiny minority of radical Islamic extremists.”

He said Ukip must remould itself as a “reasoned, radical alternative to the political establishment” or face total wipeout. “Political parties do very badly when they make it difficult for people to go out and vote for them. That’s exactly what Ukip did on Thursday,” he said.

“I’ve watched friends and family, Ukip voters, members and even past candidates, fall away from the party one after another. If Ukip is ever to resurrect itself as a serious political force, it’s going to need a good long think about what fundamentally drives people to be Ukip.”

Arnott said he would formally resign as Ukip’s general secretary and constitutional affairs spokesman after the party’s national executive committee (NEC) meeting on Monday.

Nuttall, who took over the leadership seven months ago, failed to win the overwhelmingly pro-Brexit Boston and Skegness seat as his party secured no MPs. A leadership election is under way – the party’s third in a year – with Farage considering a comeback.

Farage told the Sunday Telegraph he was “50-50” about running for the leadership but said Ukip needed to adopt the Labour model of campaigning grassroots organisation Momentum. Farage said a planned party rebrand should be dropped in favour of a restructuring that would empower party members.

“In some ways Labour has done that with Momentum, its campaign group,” Farage told the newspaper. “Labour is exceeding in online engagement in the same way. Ukip needs to go in that direction.”

Farage said he believed Ukip members should each be given a vote on whether the party should adopt certain policies in the future.