By LENKA PONIKELSKA and KRYSTOF CHAMONIKOLAS

Unipetrol shares tumbled the most in four years after a fire shut down the Czech refiner’s most profitable unit, an ethylene facility in Litvinov.

The company has halted the petrochemicals plant at the site and the adjacent refinery is operating at “a minimum level,” Unipetrol spokesman Mikulas Duda said by phone on Thursday. Unipetrol’s only otherrefinery is located at Kralupy nad Vltavou, just north of Prague.

The fire is now “under control,” Duda said. “We have evacuated all staff and so far we’re not aware of any injuries.”

A prolonged outage of the so-called steam cracker in the northern Czech city may impact Unipetrol’s earnings, according to Miroslav Frayer, an equity analyst at Komercni Banka in Prague. The refiner previously said that shutdowns of the same ethylene unit in June and July, each lasting about one week, would reduce the earnings before interest, taxes depreciation and amortization by a combined 270 million koruna ($11 million).

“Given the steam cracker is Unipetrol’s most profitable unit, this is certainly negative news,” Frayer said. “If the shutdown and repairs take more time, which is likely, this would certainly translate into a lower Ebitda.”

Shares fell as much as 8.6%, the most since August 2011, in Prague trading before paring the drop to 6.3% at 177.1 koruna as of 1:52 p.m. The price is still up 36% this year, valuing the company at 32 billion koruna.

The fire started after an explosion was triggered by a failure of the ethylene facility’s cooling system, the Litvinov city hall said on its website Thursday. While no harmful fumes have been detected in the city, the city hall recommended residents keep their windows shut and stay inside. About 1,000 people have been evacuated from the area, CTK news service reported without elaborating.

The impact of the ethylene unit outage on earnings may be mitigated by better-than-expected earnings the company reported in the first half of the year, according to Komercni Banka’s Frayer.

The maximum annual production capacity of the refinery in Litvinov is 5.4 million tons of oil, according to Duda.

Unipetrol’s majority shareholder is Poland’s PKN Orlen.