Telenor, the Norwegian telecoms group, is to appeal against a court decision ordering it to pay $2.8 billion (£1.5 billion) in damages for delaying an acquisition by VimpelCom, a Russian telecoms company in which it is a leading shareholder.

A court in Siberia found against Telenor at 2am on Saturday.

The lawsuit had been brought by Farimex, a company registered in the British Virgin Islands, which claimed that Telenor had held back VimpelCom's development when it delayed its purchase of URS, a Ukrainian mobile phone operator, for a year.

Telenor, which has a 29.9 per cent voting stake in VimpelCom, believes that Farimex is affiliated to Alfa Group, which has a 44 per cent voting stake and with which Telenor is embroiled in a series of legal battles.

“This case has absolutely no merit and there have been numerous procedural irregularities in the court's conduct of the case. We believe Farimex is affiliated with the Alfa Group and we will pursue all possible legal remedies,” Jan Edvard Thygesen, the head of Telenor's Central and Eastern European operations, said.

Telenor was found to have delayed VimpelCom's acquisition of the Ukrainian mobile operator, thereby damaging the Russian's company's growth prospects. VimpelCom bought URS in November 2005 and, while Telenor was opposed to the purchase at the time, its denies that its objections delayed the deal that long.

Since then, the Ukrainian company has struggled and it “requires regular cash injections in order to remain in business”, according to Telenor. URS has soaked up $850 million of investment and run up a further $125 million of losses, Telenor said. Its market share has fallen from 4.8 per cent a year ago to 3.8 per cent.

Telenor insisted that it was confident that it would prevail on appeal, saying that Russia's higher courts had overturned other cases involving Alfa. However, it sounded a note of caution.

While it said in a briefing paper that it “did not believe” that Farimex could seize shares or assets worth $2.8 billion to enforce the judgment, Telenor conceded - given what it said were “gross irregularities” - that it “was prepared for anything”.

The Siberian ruling came after a whirlwind hearing, in which the court sat for 16 hours on August 14 and 14 hours on August 15, leading up to the judgment in the small hours of Saturday. Assessors - who help the court to reach its decision - were appointed only on Tuesday last week.

Alfa Group, the other leading shareholder in VimpelCom, controls a range of industrial assets. Mikhail Fridman, its chairman, is worth an estimated £7 billion. Mr Fridman is one of the billionaires behind AAR, the half-owner of TNK-BP, the Russian oil group. AAR is tussling with BP for control of the business.

Telenor says that Alfa Group is linked to Farimex, which brought the lawsuit. Farimex, Telenor says, lists as its only director Dimitri Fridman, who is a cousin of the Alfa chairman.

Alfa Group denies this. But press reports that pre-date the trial identify a Dimitri Fridman as related to Mikhail Fridman. Farimex's claim also contained details about the protocol of board meetings, which under Russian law is available only to shareholders with 25 per cent or more.

VimpelCom is predominantly a mobile phone operator. Its Beeline brand operates in Russia and a handful of former Soviet states, including Kazakhstan and Georgia as well as the Ukraine. VimpelCom has 52.3 million mobile subscribers, 42 million of which are in Russia, where it has a 25 per cent share of the market. Its present market value is $24.8 billion, although its share price has fallen sharply in recent months.

Alfa Group was not available for comment last night.

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