* Advocate General says EU tobacco law is valid,proportionate
* Opinions are non-binding but often influence rulings
* Tobacco firms remain hopeful that court view will diverge (Adds comments; changes headline)
By Michele Sinner and Martinne Geller
LUXEMBOURG/LONDON, Dec 23 (Reuters) - An adviser to Europe'shighest court said an EU law on cigarettes was valid, rebuffinga challenge from Philip Morris International and BritishAmerican Tobacco, though the court still has to delivera final ruling.
The opinion, if adopted by the court in Luxembourg, would bea blow to Big Tobacco companies, which had lobbied vigorouslyagainst what was seen as some of the world's strictestanti-tobacco legislation.
In a majority of cases, opinions of the court advisers arereflected in the final ruling, which should follow in the comingmonths.
"(The advocate general) considers the EU tobacco directiveof 2014 to be valid," the opinion published on Wednesday said.
"The EU legislature did not exceed the considerable latitudeto be given to it in ensuring that tobacco and related productsmay be placed on the market under uniform conditions throughoutthe EU without losing sight of the fundamental objective of ahigh level of health protection."
Smoking is the biggest cause of premature death in theEuropean Union, responsible for nearly 700,000 deaths per year.
The EU Tobacco Products Directive (TPD) was adopted in May2014 and comes into force next year. Key provisions include thestandardisation of packaging, a prohibition by 2020 on mentholcigarettes and special rules for e-cigarettes.
The opinion also said an effort by Poland -- the EU's topmenthol producer and consumer -- to annul the menthol ban mustbe dismissed as "none of its arguments ... is well founded".
The TPD requires health warnings that include discouraging images to cover 65 percent of tobacco packages. It stipulatescigarette boxes should be rectangular with at least 20cigarettes, thereby banning slimmer 10-stick packs moreattractive to occasional smokers.
The e-cigarette rules, which the opinion called "relativelymoderate," include a duty to notify regulators six months beforeproducts go on sale, prohibitions on advertising and sponsorshipand a maximum nicotine content for e-liquids of 20 mg/ml.
PLAIN PACKAGING
The TPD also preserves the right of member states tointroduce stricter packaging rules, such as the "plainpackaging" law due to go into effect in Britain next yearbanning all colours and logos on cigarette boxes.
In so doing, Marlboro maker Philip Morris claims the TPDwould disrupt the internal market of EU.
"The directive inexplicably encourages a patchwork ofregulations and disregards important limits on the scope of EUlegislation," a Philip Morris spokeswoman said.
Philip Morris and British American, the two largestinternational tobacco firms, as well as Japan TobaccoInternational and Imperial Tobacco Group arechallenging the legality of the UK plain packaging law inseparate proceedings. A verdict is expected there in comingmonths.
Striking down the TPD would cripple Britain's plan to be thesecond country after Australia to institute the measure, aimedat reducing the number of young smokers by making the packagesless attractive.
Tobacco companies don't break out sales by country, but theysay Australia's move has been ineffective at accelerating along-term downward trend in smoking.
British American, maker of Dunhill cigarettes, said it was"obviously disappointed" with Wednesday's opinion.
"We have always maintained that the (TPD) represents anunlawful and disproportionate incursion into the autonomy of theMember States and we remain hopeful that the Court of Justicewill uphold our challenge," a spokesman said. (Additional reporting by Barbara Lewis in Brussels; Editing byKeith Weir)
