One of Europe’s most venerable human rights bodies has been warned it risks falling into irrelevance unless it sets up a robust investigation into allegations of vote-rigging in favour of Azerbaijan’s authoritarian regime.

The parliamentary assembly of the Council of Europe (Pace) has been accused of turning a blind eye to corruption, after allegations that a former senior member was paid €2.39m to engineer votes to protect the kleptocratic regime of Azerbaijan’s president, Ilham Aliyev.

Pieter Omtzigt, a centre-right Dutch parliamentarian, is urging Pace leaders to launch a “deep, thorough investigation by an independent panel” that makes its findings public.

The Council of Europe, which was created in 1949 to protect democracy and promote the rule of law, has 47 members including Russia and Turkey. Azerbaijan joined in 2001, but observers have long raised questions about the parliamentary assembly’s weak response to ballot-box stuffing and human rights violations in the oil-rich country.

Human rights groups have blamed “caviar diplomacy”, gifts of gold, silver, silk carpets and the regional fishy delicacy, which are showered on visiting dignitaries to the capital, Baku.

The latest allegations are centred on Italian politician Luca Volontè, the former chair of the centre-right group in the parliamentary assembly. He is being investigated by the Milan public prosecutor’s office for allegedly accepting €2.39m in bribes, in exchange for working for Azerbaijan in the parliamentary assembly. Human rights groups allege he played a key role in orchestrating the defeat of a highly critical report on the abuse of political prisoners in Azerbaijan in 2013. Volontè denies any wrongdoing, informs The Guardian.

The allegations, which were aired by Italian public broadcaster RAI in November 2016, have plunged the parliamentary body into turmoil.

Many senior parliamentarians have warned that failure to carry out an independent investigation would erode the credibility of the human rights body, which was inspired by Winston Churchill, and sends election monitors to every corner of Europe. “It is not credible if you tell other countries to be open and transparent if you do not investigate credible allegations of vote-rigging,” Omtzigt said.

One fifth of Pace’s 324 parliamentarians have signed Omtzigt’s resolution, which states that “recent, serious and credible allegations of grave misconduct” risk undermining public confidence in the assembly. The signatories are a cross-party coalition, drawn from 25 countries, including the UK, France, Germany, the Nordic countries, the Baltic countries, Greece and Ukraine.