LUXEMBOURG (Reuters) - An adviser to Europe's highest court in Luxembourg on Wednesday said an EU law on cigarettes was valid, rebuffing a challenge from Philip Morris International, although the court still has to deliver a final ruling.

The opinion, if adopted by the court, would be a blow to Big Tobacco, which had lobbied vigorously against what was seen as some of the world's strictest anti-tobacco legislation.

In a majority of cases, opinions of the court advisers are reflected in the final ruling, which should follow in the coming months.

"(The advocate general) considers the EU tobacco directive of 2014 to be valid, in particular the extensive standardization of packaging, the future EU-wide prohibition on menthol cigarettes and special rules for e-cigarettes are lawful," the opinion said.

Smoking remains the most significant cause of premature death in the European Union, responsible for nearly 700,000 deaths every year.

The EU Tobacco Products Directive (TPD) was adopted in May 2014 and comes into force in 2016. It requires measures including a ban on menthol cigarettes by 2020 and pictorial and text health warnings across 65 percent of tobacco packages.

It also preserves the right of member states to introduce packaging rules that are stricter, such as the "plain packaging" law the British government has passed that bans all colors and logos on cigarette boxes.

The legal challenge by Marlboro cigarette maker Philip Morris was combined with one from Lucky Strike maker British American Tobacco. Neither company was immediately available to comment.

(Reporting by Michele Sinner and Martinne Geller, writing by Barbara Lewis, editing by Robert-Jan Bartunek and William Hardy)