Fears that Vladimir Putin is trying to fill the European parliament with more pro-Russia MEPs were behind the EU’s blunt message to the Silicon Valley owner of Facebook on Tuesday.

It gave Meta just five days to explain how it will root out fake news, fake websites and stop adverts funded by the Kremlin or face severe measures.

Forty days out from the European parliamentary elections – and during a year in which countries with more than half the world’s population go to the polls – deep concerns about how Facebook is dealing with fake news were behind the warning.

“The integrity of the election is an enforcement priority,” said Thierry Breton, the commissioner for internal market, warning that the European Commission would be quick to respond if Facebook did not rectify the problems within the week.

“We expect Meta to inform us of the actions they are taking to address these risks in five working days or we will take all necessary measures to defend our democracy,” he said.

The commission confirmed it had launched formal proceedings against Meta as the clock ticks down to elections being held across Europe on 6-9 June.

The commission is extremely concerned that Russia will use Facebook, which has more than 250 million monthly active users, to try to swing votes in its favour.

As the Belgian prime minister, Alexander De Croo, said earlier this month, after a formal investigation into alleged payments by the Kremlin to MEPs, the objective of Russia is “very clear”: to help “elect more pro-Russian candidates in the European parliament”.

Officials declined to give precise examples but some are blatant, including adverts paid for by foreign agents.

“It is fundamentally wrong they [Facebook] are making money on this,” said an official.

They also say the tools to flag illegal or suspicious content are not visible enough.

Links to fake news platforms, known as “doppelganger sites”, are not being removed quickly enough or at all, the EU suggests.

Last week a Czech news agency website was hacked to display fake news including claims that an assassination attempt on the Slovak president had been foiled.

At the same time France’s Europe minister, Jean-Noël Barrot, said the country was being “pounded” by Russian propaganda with “deliberate manoeuvres to disrupt public debate and interfere in the campaign for the European elections”.

Another concern on Facebook is Meta’s decision to “suppress” discourse in an effort to de-risk user-generated content on sensitive subjects such as the Middle East.

This is known as “shadow banning”, and the EU wants Facebook to be more transparent in how it justifies these decisions.

“Users need to know about it when it happens and they need to be able to appeal it, otherwise this is a discourse risk,” said an official.

It is also concerned that Facebook was planning to discontinue a service called CrowdTangle that helped factcheckers, journalists and researchers to monitor disinformation.

Tuesday’s proceedings against Facebook are the sixth taken by the European Commission since the EU’s Digital Services Act (DSA) came into force.

But is it enough to stop the lies? Even officials in Nato, on a panel in Brussels in February, said they were treating disinformation as potent a weapon as bullets and missiles.

Officials say it is not that Facebook is “not doing anything”, it is just that the measures in place are weak, opaque and not effective enough.

Under sweeping new laws under the DSA, which came into force in August, the EU can fine social media companies up to 6% of their revenue or ban them from the union altogether.

Facebook said: “We have a well-established process for identifying and mitigating risks on our platforms. We look forward to continuing our cooperation with the European Commission and providing them with further details of this work.”