(Reuters) - A strike by labour unions that has disrupted ports and the rail system is having an impact on Finland's exports and imports.

The strike, which began on March 11 and follows others held since October, is in protest over government plans to reform the labour market and reduce social welfare, changes that the government says are necessary.

Finland's industrial, logistics and electrical workers on March 27 agreed to extend the strikes to a fourth week after a previous one-week extension until March 31.

Here is how some companies have been affected:

BOLIDEN

The Swedish metals maker estimated on March 22 the strikes' negative impact on its first-quarter operating profit at around 500 million crowns ($46.89 million).

FINNAIR

The national carrier expects disruptions to its refuelling services and planned measures to allow refuelling at stations outside Finland. It saw no impact on flights as of March 22.

KEMPOWER

The maker of charging solutions for electric vehicles cut its first-quarter guidance on March 22, citing delays to deliveries. It saw first-quarter revenue significantly below the year-ago number of 55.8 million euros ($60.33 million), and operative earnings to be "significantly negative", it said in a statement.

METSA BOARD

The forestry group Metsa shut down five mills, including Metsa Board's Joutseno mill, and said it would assess wage payments for employees involved.

Metsa also said it expected eight further mills to become idle as of March 25 because of the strike.

NESTE

The oil and biofuels group could stop production at its Porvoo oil refinery as the strike cuts off raw material transportation and leads to filling up of storage facilities.

Neste said on March 26 that fuel was no longer available at some of its stations and the situation would get worse as the strike continues.

NOKIAN TYRES

The tire maker said on March 22 it would temporarily lay off 350 employees at its car tire factory in Nokia for 90 days due to the strikes. Inventories were filling up as the strikes hampered deliveries, it said.

OUTOKUMPU

Stainless steelmaker Outokumpu on March 21 lowered its first-quarter core profit guidance for the second time.

It estimates the combined negative impact of the three-week long strike on adjusted EBITDA at about 65 million euros, a spokesperson said.

SSAB

The Swedish steelmaker estimated on March 28 that the strike would shave off about 350 million crowns from SSAB Europe's first-quarter operating result.

If the strikes continue into the second quarter, they will weaken the operating result by an estimated 125 million crowns each week, it said in a statement.

STORA ENSO

The forestry firm said on March 22 it would start shutting down its Imatra and Oulu mills on March 24 and expected them to be fully closed by the start of Easter.

Stora Enso said on April 2 it will ramp up production at its Varkaus packaging board mill after production was halted and wages suspended for affected employees in March.

It said it also expects the ramping up of its Oulu mill in the coming days.

UPM

Production has been halted at UPM's paper mills in Kymi, Rauma and Jämsänkoski and two pulp mills. However, the Jämsänkoski printing paper machine is due to restart on March 25 for five days, a spokesperson said.

The forestry company is suspending pay for employees involved.

VR GROUP

The state-owned railway, which normally runs 250 freight trains per day, said the strike has brought freight traffic to a standstill until March 31. Passenger transport will not be affected.

($1 = 0.9250 euros)

($1 = 10.6633 Swedish crowns)