Falling oil prices knocked £1bn off the value of North Sea services firm Petrofac on Monday and are on course to cripple the Russian economy without a rescue package from the oil producers’ cartel Opec later this week.

Petrofac, which operates in almost 30 countries, saw its share price fall by more than a quarter after it said profits would be low this year to reflect a 30% decline in Brent crude prices since the summer. The oil and gas services firm said it expected 2015 net profit of about $500m (£403m), $190m less than analysts had previously forecast and well below the $580m now predicted for this year.

Investors took the warning as a sign that oil majors Shell and BP along with a host of medium-sized operators in the sector would follow suit.

A barrel of Brent crude slid 40c a barrel to a fraction below $80, down from $86 a week ago. Five months of declines in the cost of oil have battered oil firms and the leading producer nations.

The Russian finance minister said the country is losing up to $140bn (£90bn) a year because of a combination of plunging oil prices and Western sanctions over the Ukraine conflict.

“We are losing around $40bn per year due to geopolitical sanctions,” finance minister Anton Siluanov said in a speech at an economic forum in Moscow, quoted by RIA Novosti news agency. The oil price was causing Russia economic damage of “some $90bn to $100bn per year”.

The European Union and the US have imposed several rounds of sanctions on Russia over its role in the Ukraine conflict, sending the rouble plunging and inflation soaring. The embargoes targeting Russia’s key energy, defence and finance sectors have been compounded by the sliding crude prices.

A lengthy dispute is expected to inflict huge damage on Moscow’s finances, which depend on oil profits for half of its revenues. Its budget for next year, passed by the lower house of parliament on Friday, is based on an oil price of $96 per barrel.

“The price of oil fell 30% since the beginning of the year – and with it the rouble,” Siluanov said. “The rouble will follow oil prices.” Russia’s currency has depreciated almost 27% this year against the dollar, the worst performance among major global currencies.

President Vladimir Putin has dismissed the economic damage as “not fatal”, insisting that the falling rouble would only hurt the Russian economy “to an extent, but not [cause] fatal” damage.

Putin has also dismissed a western blacklist of Russian officials and businessmen from his inner circle, saying the decision to impose sanctions on those individuals in an attempt to hurt him was based on a “false premise”.

Nevertheless, Russia has lobbied other oil producing nations to cut production next year to put a floor under oil prices. Moscow wants the Opec cartel of mainly middle eastern countries to agree production cuts to limit supply and counter a surge in US production from its fracked shale oilfields in Nebraska, Kansas and other mid-western states.

Without a deal at a crucial meeting later this week, producers fear the price of crude will sink further and push many of them into recession.

“The market would question the credibility of Opec and its influence on global oil markets if there was no cut,” said Daniel Bathe, of Lupus Alpha Commodity Invest Fund. That could send Brent down to around $60, Bathe said.

“Herding behaviour and a shift to net negative speculative positions should accelerate the price plunge,” he said.

Fund managers and analysts are divided over whether Opec will reach an agreement on cutting output. Bathe told Reuters the likelihood was no more than 50%.

Several countries have spent billions of dollars propping up their currencies to offset the effect of falling oil revenues. Nigeria, which narrowly escaped the Ebola crisis, has failed to prevent the naira hitting a record low despite spending almost £4bn in recent weeks buying the currency. OPEC’s poorer members, including Venezuala and Ecaudor, have also spent large slices of their foreign reserves propping up their currencies.

Russia is likely to record capital outflows of $130bn, Siluanov said, as citizens and businesses hedge against the falling rouble by converting their savings to foreign currencies. Igor Nikolayev, head of the FBK Strategic Analysis Institute, said: “Sanctions are undoubtedly a serious threat to the Russian economy. But Russia, with or without sanctions, would have experienced the crisis it is going through today.”

Energy minister Alexander Novak said on Friday Russia was considering cutting its oil production in a bid to shore up prices.

He said Russia, which is not a member of Opec, did not have the technical means to lower and raise production quickly like Saudi Arabia but that the government was studying the “expediency of such methods”.

Venezuela and Ecuador have called publicly for a cut in output, while Iran has also hinted at a need to reduce production. But the cartel’s Gulf members, led by the largest producer Saudi Arabia, have so far rejected such calls unless they are guaranteed market share, according to analysts.