The European Parliament has opened an investigation after a British newspaper reported that three MEPs had accepted offers of up to 100,000 euros per year in exchange for tabling amendments to legislation being passed in the EU assembly.

During the course of an eight-month investigation, journalists from the Sunday Times posing as lobbyists contacted some 60 MEPs to test their ethics.

The reporters wanted to verify allegations that some politicians were prepared "to sell their services" to push through specific amendments to EU legislation in exchange for remuneration, the broadsheet revealed on Sunday (20 March).

Three MEPs took the bait – Austria's Ernst Strasser, Romania's Adrian Severin and Zoran Thaler of Slovenia.

Both Severin and Thaler are former foreign ministers from the socialist family. Strasser is from the centre-right Austrian People's Party and is a former minister of the interior.

The Sunday Times writes that its spoof marks one of the biggest scandals in the 53-year history of the European Parliament.

According to the newspaper, Severin emailed reporters saying, "just to let you know that the amendment desired by you has been tabled in due time," before sending them a 12,000 euro invoice for "consulting services".

The Romanian later insisted he had done nothing that was "illegal or against any normal behaviour we have here".

"I have been offered a consultancy contract, as a member of the council of international experts of the firm Tyler Jones, a fake firm as it turned out," Severin said, speaking to Romanian agency Mediafax.

"It's a legal contract. We have the right to be members of those advisory boards. We are allowed to work as political consultants, our only obligation being not to disclose confidential information," he explained.

Severin also insisted that he had never expected to be paid, and that in any case he had not put forward the amendment which Tyler Jones had requested.

Asked by Mediafax if the report by the Sunday Times that he had asked to be paid 12,000 euros was false, Severin said: "It's not false, but it had been requested for something else […] I provided consultation services and for that I agreed to be paid."

Severin also said he had been attacked for the positions he had taken in the Parliament, which in his words had "disturbed many".

Romanian daily Gandul and various blogs published a video recording made secretly by the Sunday Times of a conversation between their reporter and Severin, in which the MEP agrees to be paid 12,000 euros for helping to pass legal amendments.

AFP quoted Parliament spokesman Jaume Duch as saying that the Parliament had just opened an inquiry into the allegations in order to fully establish the facts.

Ernst Strasser announced his resignation, while Severin said he was contemplating legal action against the Sunday Times.

For his part, Zoran Thaler sent EurActiv a written statement explaining that he realised immediately that the lobbying firm did not exist and that his further actions were intended to uncover who was behind this attempt to discredit him.

Martin Schulz, leader of the Socialists & Democrats group, is quoted as saying that it was crucial to meet both MEPs "as soon as possible, to hear what they have to say".

"A [newspaper] article is not a court ruling," Schulz said.