S&D group MEP Neena Gill says the affiliation of a controversial Serbian political party to the pan-European Party of European Socialists "should be reviewed."
It comes after a pro-Russian alliance which includes a far-right party called Dveri succeeded in winning seats in the country’s recent general election.
Dveri is openly homophobic and its leader has just recently said that sexual orientation is not a human right.
Gill and other MEPs say the case highlights growing worries over gay rights in Serbia and the rise of Dveri.
There is concern, she said, that the Democratic Party (DS), a long time Serbian affiliate of PES, encouraged their left-leaning supporters to vote for Dveri, in a move which was directly intended to help the far-right party to win seats in the new Serbian Parliament.
The Socialist MEP told this website, "I was deeply disappointed to learn that the Demokratska Stranka would have campaigned on behalf of a right-wing party that blatantly fails to recognise the rights of LGBTI people."
She added, "This is in contradiction with the values of PES, which has been at the forefront of eradicating all kinds of discrimination in Europe and bringing full equality for all minorities, including for the LGBTI community. I would urge the PES to fully investigate this case and even reconsider the party’s affiliation if needed."
In April’s election, Serbian PM Aleksandar Vucic' Progressive Party won 50 per cent of the votes, taking 131 seats in the 250-member Parliament.
The alliance that includes Dveri took 13 seats, regaining its place in Parliament.
Three separate liberal groupings entered Parliament but each scored only between five and six per cent.
Konstantin Samofalov, a spokesperson for the Social Democratic Party, said Serbia's liberals now regretted listening to the Democratic Party of Serbia and its "opportunistic" call for support for Dveri.
He said, "We believe that we could have done much, much better and come second in the polls if the Democratic Party of Serbia had joined our coalition."
The splintering of the DS, he said, undermines opposition to Vucic.
MEPs point out that the first thing Dveri did on entering Parliament was to seek a ban on Serbia's annual gay rights parade on 17 May, the International Day Against Homophobia and Transphobia.
The deputies condemned Dveri movement leader Bosko Obradovic after he said he would not support the Belgrade Pride parade and that "sexual orientation is not a human right."
Claude Moraes, Chair of Parliament's civil liberties, justice and home affairs committee, told this website, "I oppose the recent moves by the right-wing Dveri Party in Serbia to obstruct a gay rights parade. I am deeply concerned by what this party is doing to oppose LGBT rights in the country.
"At a time of increasing far-right activity we must stand up for fundamental rights and prevent discrimination. We must understand that the defence of LGBT rights is a fundamental EU value."
Greens/EFA group MEP Terry Reintke, a member of Parliament’s LGBTI intergroup, said, "Discrimination, hate speech and hate crime against LGBTI people in Serbia remains a strong concern for the Parliament.
"Human rights, including for LGBTI people, are at the heart of the European integration project. But discrimination and violence continue to threaten LGBTI people's access to basic freedoms in Serbia and the processing and investigation of such cases remains insufficient.
"Leading politicians should show the courage to condemn hatred and prevent abuse. Serbia like other countries in the region have the necessary laws in place. Now it's time to implement them. Hatred and prejudice have kept too many people not just in the Balkans from full participation in society for far too long. It's time for equal rights for all."
Another MEP, British Socialist member Anneliese Dods, said, "LGBT rights are human rights and therefore front and centre of the EU's approach toward enlargement and every country which wants to join the EU has to respect them. The LGBT community in Serbia sadly still faces hostility, discrimination and violence. Dveri's call to ban the pride is absolutely appalling and an attack on values such as freedom of assembly, freedom of expression and respect for fundamental human rights."
The NGO community has also voiced concern, with Aurelien Mazuy, President of Rainbow Rose, the PES LGBTI associate organisation, commenting, "These recent developments are very worrying. We have initiated some dialogue with our sister party on LGBTI topics which was going in the right direction. Nevertheless, Rainbow Rose will seek to get clarifications as soon as possible on this specific point. Above all, this demonstrates that LGBTI people remain a perfect scapegoat for populist and extremist political parties all over Europe.”
The group may raise the issue at a PES meeting with its sister organisations later this week.
Further criticism came from Willy Fautre, director of the Brussels based Human Rights Without Frontiers, a human rights NGO, who told this website, "No democratic political group in the European Parliament should ever compromise with EU’s fundamental values. Beyond the protest of some Socialist MEPs against the electoral alliance of the Serbian Democratic Party with the anti-LGBTI Serbian Party Dveri, the Socialist Group should clearly and unambiguously cancel the affiliation of the controversial party to pan-European Party of European Socialists."
The situation of LGBT population in Serbia remains difficult and Serbian Commissioner for equality, Brankica Jankovic, recently said that, "any discrimination, including on grounds of sexual orientation and gender identity is prohibited by law and cannot be tolerated."