LONDON (Reuters) - Britain's GlaxoSmithKline has asked its shareholders to vote at a meeting on Dec. 18 on its proposed major deal with Switzerland's Novartis, which will see the two pharmaceutical group trade more than $20 billion (12.78 billion pounds) of assets.

The transaction, which includes GSK buying Novartis' vaccines business, Novartis purchasing GSK's cancer drugs, and the two groups tying up in consumer healthcare, was unveiled in April.

In a letter to shareholders endorsing the deal, GSK's chairman Christopher Gent said approval would strengthen its franchises in vaccines and consumer healthcare to complement its leading position in respiratory and HIV.

"This is the most significant transaction for the Company since the creation of GlaxoSmithKline in 2000," he said.

(Reporting by Paul Sandle; editing by Sarah Young)