In a communiqué sent to both Ansip and fellow MEP Yana Toom (Centre), the Supervisory Committee on Party Financing (ERJK) said it had received information about an ALDE social media advertisement endorsing both candidates, ERR's online news in Estonian reports.

Although Ansip and Toom are from opposing parties in Estonia, they both sit with ALDE, now known as Renew Europe, as MEPs sit with a political grouping in the European Parliament.

The ad (see screenshot below) appears to have been paid for by ALDE, the ERJK said.

"Based on information available to you and the applicable political party law in Estonia, paying ALDE for an advert may be considered a potential violation, as it consists of a legal entity donation. This is prohibited by law and must be refunded to the donor," ERJK chief Liisa Oviir (SDE) wrote in the letter to the politicians.

Ansip responded by saying he had made no payment, either to ALDE, the Reform Party, or to any ad agency.

"Since I have not paid for the advertisement you have mentioned, it is not possible to submit proof of any payment," Ansip wrote, adding that the ad was nothing to do with him and he was unaware of its existence up to now.

"I first learned of its existence today as I was reading your letter. I have received information from Didrik de Schaetzen, head of the ALDE Communications Department, that the advertisement came on their initiative and that ALDE had €10.95 for the advertisement on behalf of Reform. In so doing, ALDE did not inform either myself or the Reform Party of the ad's publication either before or after it took place.

"I did not order this ad, or accept any prohibited donation, and for this reason I will not pay anyone for a prohibited donation of €10.95," he added.

Ansip won a seat at the European Parliament and thus had to relinquish his post as European Commissioner ahead of the end of his term in October.

Yana Toom's response to the ERJK has not been reported.

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