By Carla Mozee, MarketWatch

LONDON (MarketWatch) -- U.K. stocks gained Wednesday, as a jump in shares of oil major Royal Dutch Shell PLC helped the FTSE 100 score a win on the last trading day of the month.

The FTSE 100 rose 0.2% to 6,780.03, marking a third straight session of gains. The benchmark finished the month with a 2.8% advance, its best April performance since 2009.

The market managed to hold to gains as Europe's highest court dismissed Britain's challenge to a tax on financial transactions, a proposal backed by the European Commission in 2011.

"In this case, the court finds that the contested decision does no more than authorize the establishment of enhanced cooperation, but does not contain any substantive element on the [financial transaction tax] itself," the European Union Court of Justice said.

Royal Dutch Shell shares (RDSB) were among the strongest gainers on the FTSE after the oil producer said it plans to pay a dividend of 47 cents for each ordinary share, a more than 4% increase from last year's payment of 45 cents a share. Class B shares ended 3.7% higher and Class A shares rose 2.9%.

The oil group was the best-performing sector, as shares of Tullow Oil PLC also advanced, by 1.7%. Tullow said it sold a stake in two of North Sea assets it was looking to sell for $75.6 million. It also backed this year's production guidance of 79,000 to 85,000 barrels of oil equivalent a day.

Reassurance from copper producer Antofagasta PLC that its full-year production guidance remains on track helped drive up its shares by 0.7%.

But GlaxoSmithKline PLC shares suffered a 2% loss to lead decliners on the FTSE after the drug maker posted a 30% drop in first-quarter profit. Glaxo in recent days said it's selling its oncology unit to Novartis AG for $14.5 billion, and plans to combine its consumer divisions with Novartis's.

Standard Life shares lost 1% after the insurance company said U.K. annuity sales have declined about 50% since the government proposed pension-industry changes in March.

Also, British American Tobacco PLC fell 2% as the maker of Lucky Strike and other cigarette brands posted a 12% decline in first-quarter sales, hurt by a fall in exchange rates in emerging markets.

While foreign exchange "remains an issue for reported results, it is a good start to the year," the company said.

Meanwhile, Saga Group said it plans to raise 550 million pounds ($925.5 million) in an initial public offering of at least 25% of its shares on the London Stock Exchange. Such a listing would value the company, which sells insurance and other services to people over 50, at GBP2.2 billion.

Next week, investors in U.K. assets will watch for the European Central Bank's policy statement, which will follow preliminary data from Eurostat on Wednesday showing consumer price inflation in the euro zone rose by a slower-than-expected rate of 0.7% in April. The euro zone is a key U.K. trading partner.

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