By Ben Hirschler

LONDON (Reuters) - GlaxoSmithKline's experimental platelet-boosting drug Promacta has been granted priority review by U.S. health regulators, boosting prospects for its early launch, Europe's biggest drugmaker said on Monday.

A priority review means the U.S. Food and Drug Administration is likely to decide within six months whether to approve the treatment, faster than the usual 10-month review period.

Glaxo, which views the medicine as a potential blockbuster, said it also planned to submit a marketing application in 2008 for the drug in Europe, where it will be sold as Revolade.

Shares in the group were 1.2 percent higher at 11.15 pounds in a sharply weaker London stock market by 2:25 p.m.

Nomura Code analyst Mike Ward, who sees sales potential of well over $1 billion (500 million pounds), said the decision was good news, since the presence of a rival experimental drug from Amgen Inc meant a priority review had not been certain.

"It's a nice tick in the box for Glaxo," he said.

The drug, known generically as eltrombopag, was discovered as a result of a research collaboration between Glaxo and Ligand Pharmaceuticals Inc and is a key new product for the British-based group, which faces declining sales of older drugs.

Ligand stock jumped 8 percent to $3.66 in early U.S. trade.