Russian potash miners Uralkali and Silvinit are set for a new legal challenge to their merger from fertilizer group Acron, which wants a better deal for its small Silvinit stake.

Uralkali and Silvinit -- both majority owned by billionaire tycoon Suleiman Kerimov and associates -- agreed merger terms at a shareholder meeting earlier this month.

The combined group would be the world's second biggest producer of potash, a key ingredient of fertilizer used in farming, creating a Russian national champion in a global industry.

Acron has long opposed the deal and unsuccessfully sought a court injunction to postpone the crucial shareholders' meeting that approved the merger in early February.

It believes the terms of the deal are stacked against minor shareholders in Silvinit and strongly favour those in Uralkali.

"(Acron is) again attempting to challenge the share conversion ratios duly approved by the shareholders of both companies and (is) seeking an injunction to block the merger-related procedures," Uralkali said in a Monday statement.

"Uralkali and Silvinit believe that the claim is entirely without merit and intend to contest it vigorously," it added.

Acron's stance was backed late last year by Moscow's Investor Protection Association and many analysts. Ill treatment of small shareholders has long been seen as a deterrent to overseas investors in Russia, who consider corporate governance standards at many companies to be unacceptable.

Uralkali said Acron and three investment groups had filed a claim against the two potash companies in a court in Perm, a city east of Moscow close to Uralkali's home base of Berezniki.

The claim is seeking to scrap the decision to approve the merger, overwhelmingly supported by other shareholders and expected to be completed in May. A spokeswoman for Acron declined to comment.

Russian business daily Vedomosti reported earlier this month that Acron's Chief Executive Ivan Antonov had asked Prime Minister Vladimir Putin to stop the deal, although Kerimov is known to be an ally of Russia's most powerful politician and is likely to have secured his backing.