A poster plastered on to a church wall compares Ukip leader Nigel Farage to far-right despot Adolf Hiter and British extremists Oswald Mosley and Nick Griffin, the former BNP leader.

The mural-style image featuring Farage at the front sparked outrage in Ukip – but has been defended as valid by a former minister in the Church of Scotland.

Ewan Aitken insisted the divisive image posed a valid question about Ukip's policies. It went up at St John's Church in Edinburgh last weekend. But Ukip MEP David Coburn condemned the poster.

The political party has been dogged by Nazi controversies, including when an MEP praised Hitler for his public-speaking skills to a room full of young party members. Also, Farage was forced to deny singing Hitler Youth marching songs as a youngster.

Admitting the poster "pushed boundaries" Aitkens said: "Whether or not you agree with the composition of the murals subjects, there are questions to be asked about a political party whose policies predicate change rather than celebrating differences.

"I think the church needs to be speaking outside its walls and challenging people in unexpected ways, on unexpected subjects."

Unsurprisingly, Coburn – the only elected Ukip politician in all Scotland – was not impressed by his leader being likened to Griffin. The former leader of the nationalist BNP has vowed he will vote Ukip at next year's general election.

"This is not going to do them any good, it is insulting all those Scottish voters," Coburn said. "It is insulting to Nigel Farage [and] is possibly libellous.

"We [Ukip] have a sense of humour, but it's not that funny. It's rather insulting [and] beyond a wee bit of fun."