Chief Prosecutor Sotir Tsatsarov ordered an investigation into media reports alleging improprieties surrounding real estate bought by Plamen Georgiev and his wife, his office said in a statement.

Investigative website bivol.bg last week claimed there were serious discrepancies between the property declarations and the official income of Georgiev.

A luxury three-level apartment in Sofia acquired by Georgiev in 2017 was also bought at below market price, bivol.bg said.

Besides Georgiev, the National Investigation Service chief's son and the wife of the chairman of the Supreme Court of Cassation also came under investigation Thursday over their property purchases.

Conservative Premier Boyko Borisov on Thursday called on "everyone embroiled in these scandals now to also resign or take a long leave during the course of the checks".

Georgiev, whose committee is subject to parliamentary control but is independent from the government, has agreed to take leave. He insists he did not do anything wrong, describing himself as a victim of slander.

As chief of the independent commission for illegal assets forfeiture, Georgiev has been tasked with checking for any discrepancies between the official income and the property of top public servants and politicians.

Similar findings by bivol.bg about posh property purchases at below market prices already prompted the resignations over the past weeks of the justice minister, two deputy ministers and a high-ranking official of the ruling GERB party.

The scandal, dubbed "Apartment Gate" in the media, has been a severe blow to Borisov, shrinking his cabinet's approval rating to just 13 percent, according to a recent Alpha Research poll.

Sixty-seven percent of those polled said the apartment scandal was an example of "abuse of power" and 57 percent said "nobody battles corruption".

Ahead of the European Parliament elections at the end of May, Borisov's GERB party is polling at 33.9 percent with the main opposition Bulgarian Socialist Party (BSP) edging closer on 33.4 percent, Alpha Research said.

Real estate agents have commented that Bulgarians declare lower prices in order to pay less taxes but added that the practice might also serve money-laundering purposes.