Ukip is facing a fresh storm of controversy as further evidence emerged of racism among its local election candidates, including a suggestion by one that the comedian Lenny Henry should emigrate to a "black country".

Candidates have taken to social media sites to rail against Islam as "organised crime under religious camouflage" while likening the religion to Nazism, and suggesting that the murder of the black teenager Stephen Lawrence has received a disproportionate level of attention.

One candidate for election in Enfield, William Henwood, responded to a recent speech by Henry, in which he suggested there was a poor representation of black and ethnic minorities on British television, by tweeting: "He should emigrate to a black country. He does not have to live with whites."

Ukip, polling at 18% in the latest Observer/Opinium findings, said the party was "non-racist, non-sectarian" and that "any comments made by members that fail to uphold these values will be duly investigated and acted upon".

The latest developments will only add to Nigel Farage's woes after a disastrous week for the party in which its big advertising campaign has been picked apart. Ukip has already been forced to suspend the "poster boy" of its European party election broadcast after it was revealed he had posted a series of racist comments on Twitter. A builder in a Ukip poster accusing EU workers of taking UK jobs was discovered to be an Irish actor who migrated to this country, and a woman who appeared on a poster as a voter from Devon was Ukip's events manager and an assistant to the party leader.

Ukip has attempted to improve its vetting of candidates in the local and European elections to be held next month, but it appears that some of their members will not be constrained on social media. Along with his comments about Henry, the party's candidate in Enfield Town, London, has tweeted that "Islam reminds me of the 3rd Reich, strength through violence against the citizens". He said Muslims "like us to fawn to them" and "young Muslim men remind me of young Afrikaners. They are taught at an early age they have the right to abuse".

A Ukip candidate in Camden, Magnus Nielsen, used his Facebook account to post: "70% of mosques in the UK have been taken over by Wahabbi fundamentalists. Islam is organised crime under religious camouflage. Any Muslim who is not involved in organised crime is not a 'true believer', practising Islam as Muhammad commanded".

And, earlier this month, in the wake of allegations of police corruption in the investigation into the murder of Lawrence, Matt Pavey, a Ukip candidate in Lewisham, tweeted: "Does anyone remember the name Jean Bradley murder unsolved in Acton, London, in 1993. Anyone looking for corruption here? No, thought not." Pavey tweeted two days later: "Lets all take a moment to remember Jean Bradley stabbed to death on the streets of London in March 1993 and no justice #stephenlawrence." And again that day he tweeted: "Lets please take a moment to remember Penny Bell stabbed 50 times in London 1991. No justice #stephenlawrence." Pavey said he was not prepared to comment on his tweets but that there was "no malice" intended.

Gareth Thomas, the shadow minister for Europe, said the tweets should be seen as evidence that Ukip is "not fit to represent the country in Europe".

The Observer has also learned that Ukip MEPs are employing foreign nationals as parliamentary assistants while the party publicly takes a tough public stance against European immigrants taking British jobs. Last week Farage was involved in an exchange with the BBC's political editor, Nick Robinson, over the employment of his German wife as his secretary.

Gerard Batten, Ukip MEP for London, has hired foreigners in both his European and UK constituency offices. Pavel Stroilov, from Russia, works in Batten's London office, while a Polish citizen, Kamila Zarychta, is his accredited assistant in Brussels. Batten said: "I employ people on the basis of whether I think they can do the job."

Ukip's East Midlands MEP Roger Helmer, who has an Italian assistant, Francesca Salierno, in Brussels, said: "I absolutely reject the assumption that because we're opposed to mass immigration it is therefore somehow inconsistent and hypocritical for a Ukip MEP to hire a foreign person." he said William Dartmouth, Ukip MEP for the south west of England, employs Line Sophie Munk Olsen, a Dane, as one of his four parliamentary assistants. A spokesman said: "He employs four British people in his office as well."