Post, a Socialist member, has demanded an investigation into the alleged comments by Angel Dzhambazki, a Bulgarian MEP.
She details the complaint in a letter to European Conservatives and Reformists group Chair Syed Kamall, who is joint President of the European Parliament Anti-Racism and Diversity Intergroup (ARDI).Dzhambazki is also a member of the European Conservatives and Reformists group.
The ECR group was recently rocked by allegations that another of its members, German MEP Marcus Pretzell, had allegedly advocated shooting refugees at Europe's borders. He later distanced himself from the remarks but was thrown out of the group.
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In the letter to Kamall, published on the ARDI website, Post, who has worked with Roma people in the past, says she wishes to draw his attention to a racist, xenophobic and homophobic speech by Dzhambazki.
Post writes, "Dzhambazki has in the past few weeks openly called for a civil war against Roma in Bulgaria in an article authored by him as well as engaged in hate speech online against Roma people, Muslims and LGBTI people."
She tells Kamall, a British Conservative MEP, "Therefore I would urge you to take immediate action against Dzhambazki. Such behaviour which is clearly hate speech should not be tolerated and should be sanctioned by the ECR group.
"His actions are all the more irresponsible and dangerous given that he is an MEP and the current context in Europe where the rights of non-discrimination and equality have come under attack.
"Racism and xenophobia are direct violations of the principles of liberty, democracy, respect for human rights and fundamental freedoms and the rule of law," writes Post, a member of the civil liberties, justice and home affairs committee.
Post represents the Swedish Feminist Initiative party. Her father was a German-born Jew and her mother was a Romani.
In the letter, Post, who is co leader of the intergroup, concludes, "It is imperative that all of us fight intolerance and I hope you take action."
On Friday, an ECR spokesperson told this website, "We have received the letter from Ms Post and will look into it."
Dzhambazki is the Vice-Chair of the IMRO, the Bulgarian National Movement party, having joined the nationalist party in 1997 and gradually progressed through its ranks.
He is a member of Parliament's delegation to the EU-Former Yugoslav Republic of Macedonia joint parliamentary committee
It is not the first time his alleged inflammatory remarks have landed him in trouble.
In November 2013, the Bulgarian Helsinki committee complained to the chief prosecutor of Bulgaria about comments said to be made by Dzhambazki during an anti-immigrant procession in Sofia.
In an online interview in 2014, Dzhambazki was asked if there was one thing he could change about the EU what that would be.
His reply was, "The resolution for the human rights of the third gender is absolutely an unacceptable targeting of humanity against nature. There are men and there are women in the normal world, everything else is a preserve liberty. And this craziness must not be regularized in any manner."
The 36-year-old Sofia-born MEP, who sits on the committee on education and culture, was not immediately available for comment on Friday.