Labour MP Gisela Stuart, who is from Bavaria and moved to Britain in 1974, was representing the Leave campaign alongside Boris Johnson and Tory minister Andrea Leadsom in a BBC debate in front of 6,000 people at Wembley Arena.
But businessman and Remain backer Lord Sugar took exception at her appearance in the debate, tweeting: “I find it strange that Gisela Gschaider [referring to her maiden name] a 1974 immigrant from Germany is on the Brexit panel telling us British what we should do.”
Tory MP and Leave backer Nadhim Zahawi hit out at his remarks, saying: “You must withdraw and apologise to Gisela. That is disgraceful. She is as British as you are.”
And former Tory MP Louise Mensch tweeted: “@Lord_Sugar PIG”.
Others piled in to take exception to his tweet, calling it "racist" and "nonsense".
Lord Sugar later denied he was being racist, pointing out that Ms Stuart is an immigrant who is now arguing against immigration, the Press Association reported.
In a series of tweets, he said: “Yes she is (British) now, and has been arguing that we need to curb immigrants from the EU. She immigrated to UK from Germany in 1974.
“30 mins ago was advocating that we stop immigration from the EU like Germany in future and exit EU.”
Retweeting someone who called him racist, the peer said: “You ignorant fool. How can my comment be classified as racist. Crawl back under your rock."