David Wycherley is standing for Ukip candidate for the Rushall-Shelfield ward in Walsall. Photograph: Facebook

A fresh Ukip candidate was embroiled in a row over racism on Tuesday when it was discovered he had taken to Facebook to argue that Team GB Olympic star, Mo Farah, isn't British because he is an "African".

The revelation came as a Ukip candidate who had urged Lenny Henry to "go back to a black country" left the party and two other members were suspended in a bid to clean up the party's image. Nigel Farage claimed the party may have been infiltrated by people trying to bring it down.

David Wycherley is standing for Ukip candidate for the Rushall-Shelfield ward in Walsall. Photograph: Facebook

But simultaneously it emerged that David Wycherley, standing for Ukip candidate for the Rushall-Shelfield ward in Walsall, had asked his Facebook friends to explain "how Mo Farah, an African … has won a Gold medal for Great Britain".

Farah, who grew up in Hounslow in London, became a double Olympic champion for Team GB at the 2012 Olympics and was later awarded a CBE by HM the Queen for services to athletics.

In other comments Wycherley joked about "starving Africans" whilst complaining about his water bill.

The party announced on Tuesday that candidate William Henwood had resigned after he urged the comedian Lenny Henry to go back to a "black country". A Ukip spokesman said Henwood had resigned his membership of Ukip after it was "mutually agreed this would be the best course" acknowledging that his comments had "caused enormous offence".

Ukip also confirmed it will expel two unnamed members, a former member of the British National party and a donor to the English Defence League.

Ukip has resisted, expelling MEPs and councillors over controversial remarks about Muslims, gay people and women in the past.

However, it appears to have acted against the two members because Farage imposed a ban on former members of the BNP and EDL joining Ukip.

Earlier, Farage dismissed rows about the views of candidates and members, saying "there is this idea that because one person or two people make silly comments that represents the entire party".

He also revealed suspicions that "one or two people have joined Ukip with the intention of perhaps not doing it any good".

"I'm investigating that, looking at that as we speak," he told the BBC.

Farage also hit back at a Guardian report that a cross party campaign is being launched to brand his party as expressing racist views.

Farage said: "I am really sorry that millions of people who have decided to vote UKIP next month now find themselves accused by the political establishment of supporting racism.

"This is like the incident between Gordon Brown and Gillian Duffy at the last general election writ large – this time it is not merely one person being slandered by one establishment party but huge numbers of decent British people under attack and all three Westminster parties levelling the charge of racism and bigotry. So they are trying to browbeat the British public into abandoning Ukip and sticking with open-door immigration by using the most disgraceful slurs. This is the classic tactic of any cartel whose position is threatened by a new competitor in the market place.

"When Ukip was perceived mainly as a threat to the Conservative party, it was David Cameron and his colleagues who used this tactic. Now we are recognised as a threat to the entire establishment it is all three parties that are slinging mud."