Tallink's passenger ferries which operate between Tallinn and the Finnish capital Helsinki, 80 kilometres (50 miles) apart, discharge effluent into international waters in the middle of the Gulf of Finland. Before it is discharged, the waste water is passed through a purification system.

"Tallink decided to once again carry out analysis of the quality of our vessels' purification systems," Tallink said in a statement after Finnish media raised a hue and cry over the waste discharge.

"Until the results of the analysis are clear, we have decided to discharge the purified waters in ports."

Tallink added that its large vessels have biological waste water treatment equipment, "as a result of which the water purified on board the vessels is fully safe for the environment and people", the company said.

Tallink added that international certification bureaux have issued its large vessels with Sewage Pollution Prevention Certificates, which show that waste water treatment on board the ships complies with international standards.

"The water discharged from the purifiers of the vessels is in many ways cleaner than sea water," Tallink said.

The company's decision to suspend the practice, at least temporarily, came after several Internet sites in environmentally conscious Finland called for a boycott of Tallink.

The boycott calls came after a Finnish newspaper ran an article about the Estonian company's practice of dumping waste water directly into the sea.

Tallink said it was "surprised" by the article and criticism from competitors as its new large vessels use "the most modern water treatment systems, the purification quality of which surpasses the quality of water treatment centres in many towns".

Though it is not forbidden, other shipping lines operating in the Baltic region have already abandoned the practice of discharging waste water into the sea.

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