United Company Rusal is running into problems finalising its purchase of a 25 per cent stake in Norilsk Nickel, the world’s biggest nickel miner, from Mikhail Prokhorov, the Russian billionaire, an official close to UC Rusal’s shareholders said.

The official and another person familiar with the deal said Mr Prokhorov was disputing a condition in an agreement he signed in November. As part of that deal, UC Rusal said it would buy the 25 per cent stake in Norilsk owned by Mr Prokhorov’s Onexim Group, as part of a possible future merger.