Investors organisations Eumedion and VEB have questioned the way Italian investment house Exor was able to build up a 15% stake in medical technology group Philips in one go, the Financieele Dagbad reported on Thursday.

Exor surprised investors with the announcement on Monday and became the biggest single investor in the company overnight. By law, investors are required to report significant stakes – 3%, 5% and 10% to the regulator AFM. But in the case of Exor and Philips this did not happen.

The FD said it had been told by market sources that Exor had signed a deal with Goldman Sachs to provide most of the shares – some 12% – while it bought just under 3% on the open market on Friday. The shares are likely to have come via the bank's securities lending desk, the paper said.

Thanks to the agreement with Goldman Sachs, Exor was able to avoid reporting significant stakes in the company, bringing with it both a financial and a strategic advantage, the paper said.

If this is what happened, "then we need to look at the reporting requirement critically again," VEB director Gerben Everts told the paper. "Clear insight into trade is crucial for a properly functioning capital market."

Rients Abma, from institutional investors organisation Eumedion, told the paper he too has questions about the deal. "We are checking a few things out," he said.

The fact that Exor and Philips did not state which investment bank was involved in the deal is further fuel for the fire, the FD said, adding that Goldman Sachs declined to comment on the market rumours.

The AFM said in a reaction that it was following the case closely but could not comment in detail. Exor told the paper that "all the rules and guidelines were complied with while acquiring the stake".

On Thursday morning, the AFM register was updated to include a 12.17% stake in Philips by Goldman Sachs.