Executives fear the disclosures - such companies would have to disclose consultancy and sponsorship contracts - will hamper their competitiveness.

This is not the right way to strengthen transparency; the requirements should extend to all firms if there is a desire for such control, said Borut Jamnik, the boss of the state-owned pension insurer Modra Zavarovalnica.

The idea was also endorsed by Tomaž Kuntarič, boss of the fund SOD, which manages state capital stakes in dozens of companies.

The bill is based on "insulting assumptions". "Not everything that is transparent should be made public," he said.

Similarly, Andrej Slapar, CEO of insurer Zavarovalnica Triglav, said the legislation did not sufficiently deal with trade secrets and would put his company at a disadvantage.

"We wouldn't have problems with it if this applied to all insurers. Disclosure of basic information from consultancy agreements could cause us grave harm," he said.

These arguments were rejected by Information Commissioner Nataša Pirc Musar, one of the driving forces behind the legislation alongside the Commission for the Prevention of Corruption.

"I have the feeling even Prime Minister Alenka Bratušek...knows that she is no longer in charge in this country, lobbies are," she said.

Pirc Musar also accused the executives of not understanding what transparency means in the first place.

Under the new law, the companies concerned will have to disclose information on the value of all transactions and the counter-party, unless that could undermine their competitive advantage.

Regardless of that exemption, they will have to disclose all information about donations, sponsorships, consultancy agreements and contracts for copyrighted works. Data on executive pay will also be subject to disclosures.