Nato countries have stepped up patrols to protect critical underwater infrastructure in the Baltic Sea, which is bordered by eight countries, and other waters. A Royal Navy submarine was ordered to surface last November close to a suspected Russian spy ship which was loitering over undersea infrastructure in UK waters.

Swedish authorities boarded a Maltese-flagged ship seized in connection with the latest breach of cables running along the bottom of the Baltic Sea to begin an investigation into the matter, the country's security police said on Monday.

Estonia’s defence minister proposed on Wednesday that shipping companies may have to pay a fee to use the Baltic Sea, one of the world’s busiest shipping routes, to cover the high costs of protecting undersea cables following a series of breaches,