Former Bulgarian Prime Minister Ivan Kostov has asked the head of a Burgas-based Lukoil refinery for access to import contracts of crude oil.

The latest development comes as part of a dispute between Lukoil Bulgaria and Kostov's think-tank, the Risk Management Lab, which alleges that the Russian oil giant has been manipulating oil prices in the country, affecting households and business competitiveness alike.

In turn, Lukoil has threatened to sue Kostov over spreading false information that is damaging to the country's business.

The claims are based on figures about differences in prices of the Urals crude, with Lukoil supposedly writing them up while importing the crude into Bulgaria.

According to the news website Dnevnik.bg, the former PM asked the Chair of Lukoil's Managing Board, Sergey Andronov, to hand out import contracts dated between 2006 and December 31, 2014.

He had earlier told Offnews that the Risk Management Lab would take note of any instance of incorrect information pointed to by Lukoil or to "anyone who would like to scrutinize our calculations and analyses".

In his words, the Risk Management Lab needs Lukoil's import contracts to verify them with the information provided by Bulgaria's Customs Agency and either find out that the latter is correct or point more precisely to the difference in prices.