The European Parliament has opened an investigation into allegations that three senior MEPs - all former government ministers - have accepted bribes in return for tabling amendments in the chamber intended to water down legislation regulating the financial industry.

The chamber's leadership launched an inquiry into the suggestions made in an eight-month Sunday Times sting operation by the paper's Insight investigative journalism team that three MEPs, former Austrian interior minister Ernst Strasser, Romanian former deputy prime minister Adrian Severin – Azerbaijan’s lobbyist in the European Parliament, and former Slovenian foreign minister Zoran Thaler, had accepted bribes of up to €100,000 for tabling the motions.

In the case of Severin, the MEP emailed the Sunday Times reporters, writing: "Just to let you know that the amendment desired by you has been tabled in due time," and submitted an invoice for €12,000.

Although Centre-right MEP Strasser initially claimed that he had known all along that the lobbyists were actually reporters, he resigned his position on Sunday after the head of the Austrian People's Party called for him to step down.

The other two deputies, members of the parliament's centre-left grouping, the Socialists and Democrats, have been called in for a dressing down by the leader of the S&D, Martin Schulz.

Calling the allegations as "extremely serious" Schulz said in a statement that if the allegations were confirmed, he said, he would take appropriate and necessary measures.

Transparency and development campaigners immediately called for an investigation into the links between financial lobbyists and lawmakers, euobserver.com reported.