Meszaros, the mayor of Orban's home village and regarded as a close friend of the prime minister, also has stakes in the media and banking sectors where the premier has a stated goal of boosting the influence of local investors.

Hungary's Russian-backed plan to expand its Paks nuclear power plant got the green light from European Union regulators in March, giving final clearance to the 12-billion-euro project, which Orban has called "the deal of the century."

Acting as a sub-supplier, Brno-based Kralovopolska Ria has participated in the construction of both nuclear blocks of the Temelin nuclear power plant in Czech Republic as well as nuclear projects in Slovakia, according to information on its website.

Its Hungarian subsidiary, KPRIA Magyarorszag, deals with engineering and construction, the production of engines, turbines and electronic power generators, company records show.

Opimus, Konzum and KPRIA aim for further expansion in the central European region in power plant development, including nuclear, and water management projects, the companies said in a statement published via Hungarian state news agency MTI.

Officials for Kralovopolska Ria and KPRIA were not immediately available for comment.

Hungarian Opimus Group, in which Meszaros is the biggest shareholder with a 16.9-percent stake, said it had acquired a 40-percent stake in KPRIA Magyarorszag. Financial terms of the deal were not disclosed.

Holding company Konzum, in which Meszaros controls a 19.6-percent stake, owns a further 11 percent in KPRIA following the transaction, it said in a separate stock exchange filing.

At 1424 GMT, Opimus shares were 8.7 percent higher, outperforming the wider Budapest stock index, which gained 0.4 percent. Konzum shares gained 1.9 percent.