* Shippers to begin monitoring from 2018

* Environmental groups say law is weak, shippers favourable

BRUSSELS, Nov 26 (Reuters) - The shipping sector will for the first time have to monitor its carbon emissions under a law agreed by the European Union on Wednesday, intended as a step towards tackling a growing source of pollutants linked to climate change.

International shipping accounts for around 3 percent of the world’s emissions of carbon dioxide, a share which could increase to 18 percent by 2050 if regulation is not in place, according to the International Maritime Organisation (IMO).

The law stops short of including shipping in the EU’s Emissions Trading System (ETS), the bloc’s flagship tool for cutting pollution, but EU officials said it was a step in that direction.

Gian Luca Galletti, environment minister for Italy, current holder of the EU presidency, said the agreement had “a great political value as well as technical”.

It introduces a mechanism for monitoring, reporting and checking shipping emissions as a follow-up to an October EU agreement on new targets to tackle climate change and a precursor to U.N. efforts to seal a global pact in 2015.

Speaking on condition of anonymity, diplomats said four nations - Greece, Cyprus, Malta and Poland - had voted against the law, but that had not been enough to block it. The measure was expected to be signed into law, pending formal publication over the coming months.

Years of debate in the IMO and at the U.N. have failed to come up with a way of including shipping emissions in mandatory pollution curbs.

Environmental campaigners say the new EU law is weak because it only measures emissions, although they still welcomed it as progress towards tougher steps.

Shippers, such as Denmark’s A.P. Moller-Maersk A/S , the group behind the world’s biggest container shipping operator, said the new law was a pragmatic solution.

The law has a long lead time as ship-owners will only have to monitor emissions from Jan. 1, 2018, and only from ships of more than 5,000 gross tons.

In the event an international deal is reached to reduce shipping emissions, the European Commission, the EU executive, will have to review the EU law. (Editing by David Holmes)