More than 1,000 sacks of pellets, known as nurdles, are believed to have fallen from the Toconao, operated by Danish company Maersk, on 8 December.
Hundreds of volunteers have been working to clean up the spill in the north-west Galicia region.
The alarm has also been raised on the Asturias coast, further east.
As many as six containers are believed to have fallen from the Liberian-flagged Toconao some 80km (50 miles) west of Viana do Castelo in northern Portugal. Of these, one contained at least 26,000kg of pellets, while the others were carrying goods such as clingfilm, tyres and tomato sauce.
Dozens of coastal communities have seen a “white tide” of pellets gradually washing up ashore since 13 December.
The worst-affected areas are around the Galician port town of Noia and fishermen further south in Vigo have been looking out for sacks of pellets floating in the Atlantic, although recent heavy seas have made that task harder.
Public prosecutors have opened an investigation and there are fears that the spill could soon spread further east along the northern coast towards the Basque country.
In a statement, Maersk spokesman Rainer Horn said the shipping company regretted the incident and would investigate.
The tiny plastic balls – used to manufacture common goods such as plastic bottles – are less than 5mm wide, making cleaning up extremely difficult. Volunteers have been combing through sand and sieving water to find the plastic pellets.
The developing plastics crisis is reminiscent for local communities of Spain’s worst ever environmental disaster in 2002, when the oil tanker Prestige broke up off just off the Galician coast, spilling more than 60,000 tonnes of heavy fuel oil along the region’s shoreline.
Authorities say the pellets, which are made of PET plastic, are non-toxic. Still, there are fears the sheer scale of pollution may endanger wildlife, the environment and pose a risk to the fishing industry in the area.
The Ecologists in Action group has said it will file a complaint against the Danish shipping company.
The nurdles can be ingested by animals and then contribute to plastic pollution in the food chain, including for humans.
PET is non-biodegradable and any pellets that are not cleaned up will remain in the environment for centuries.
Around 300 million tons of nurdles are manufactured every year. Some 230,000 tonnes are believed to end up in the oceans.
In 2017, an estimated 2.25 billion nurdles spilled from a ship moored in Durban, South Africa.
Three years later, 10 tonnes of pellets fell into the sea off the coast of Germany when a container on the MV Trans Carrier was damaged in a storm.
Nurdles from the South African spill ended up as far afield as Western Australia, 8,000km away.
