Godfrey Bloom has hit out at Nigel Farage for scrapping Ukip's 2010 manifesto, saying the party has adopted a "no-policy policy".
In a strongly-worded warning to his former colleagues, the independent MEP said the party was turning into a "don't-frighten-the-horses, all-things-to-all-men, pale blue party".
Writing for Politics.co.uk, Bloom lashed out at the party leadership for branding its 2010 manifesto "drivel" and undertaking a professionalisation drive aimed at getting so-called 'Walter Mitty' characters out of Ukip.
"The current lurch to the no-policy policy will damage Ukip in 2015," he wrote.
"The EU election is for putting the boot into the muppets who now govern us. But in 2015 the party will have to put some credible items in the shop window.
"Ukip are now getting modest, grudging praise from the Observer and Independent. They have come into the mainstream; respectable almost. But being invited to drinks by Polly Toynbee cuts no mustard in Leeds or Hull.
"Activists who joined in despair from the modern Conservative party came to Ukip out of conviction, not for a 'don't-frighten-the-horses, all-things-to-all-men, pale blue party'."
The Yorkshire and North Lincolnshire MEP, who lost the Ukip whip after he called a room full of women "sluts" and hit a broadcast journalist with a rolled up magazine, said the party was in danger of losing the qualities that had allowed it to surge in opinion polls.
"Ukip has an opportunity to break the mould: a truly radical and new party to offer something different to a tired, disillusioned and disengaged electorate," he wrote.
"Is Ukip to be a sort of Tory party, a Thatcherite, eurosceptic think tank which will disappear if the Conservative retrieve their brand?
"Ukip does have some of the wrong people at the moment. And I think some of them dwell in Ukip's central office."
Farage promised a beefed-up screening system for Ukip candidates this week, in a bid to draw a line under a stream of controversial comments from party officials.
Ukip councillor David Silvester recently said extreme weather was the result of the same-sex marriage bill while former Ukip donor Demetri Marchessini paid the Telegraph for a bizarre "open letter" to columnist Libby Purves branding homosexuality an "abomination".