(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, the head of trade union association SAK said, following a previous one-week extension until March 31.

Here is how companies have been affected:

BOLIDEN

The Swedish metals maker said on March 22 it sees a negative impact of around 500 million crowns ($47.6 million) on its first-quarter operating profit due to strikes in Finland.

FINNAIR

The national carrier expects disruptions to its refuelling services and planned measures to allow refuelling at stations outside Finland.

According to the latest information, there will be no impact on flights as of March 22, the carrier said.

KEMPOWER

The maker of charging solutions for electric vehicles cut its first-quarter guidance on March 22 as the strikes are affecting transportation and causing delays to deliveries.

It estimates first-quarter revenue to be significantly below the year-ago number of 55.8 million euros ($60.37 million), and operative earnings before interest and tax (EBIT) to be "significantly negative", it said in a statement.

METSA BOARD

Forestry firm Metsa group shut down five mills, including Metsa Board's Joutseno mill, and said it will assess wage payments for employees involved.

Metsa group said on March 21 that production will also stop at four of its sawmills on March 22, and estimates four other mills would stop on March 25.

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 storage facilities being full.

Neste said on March 26 that fuel is no longer available at some of its stations and the situation will 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 have meant it cannot deliver to customers, 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 to be about 65 million euros of which the additional week is about 25 million euros, a spokesperson said.

SSAB

The Swedish steelmaker said the strike would reduce SSAB Europe's first-quarter operating result by some 300 million crowns.

The prolonged strike will disrupt activities at SSAB Europe's Raahe plant in Finland.

STORA ENSO

The forestry firm halted production at its Varkaus packaging board mill and suspended wages for affected employees.

Stora Enso said on March 22 it would start fully shutting down its Imatra and Oulu mills on March 24. It expects them to be fully closed by the start of Easter, the spokesperson said.

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 = 10.5042 Swedish crowns)

($1 = 0.9243 euros)