Glaxo does not expect to apply for non-prescription status for its rival drug, Zantac, until the end of 1994.
Tagamet, which also relieves symptoms of dyspepsia and hyperacidity, has been available on prescription in Britain since 1976.
Yesterday's decision by the Committee on Safety of Medicines brings the drug a crucial step closer to over- the-counter status.
The markets closed before news of the decision came through, but a favourable reaction is expected this morning. Analysts said the decision could indicate that Tagamet will win similar status in the US soon.
Recent figures from SmithKline estimated the total US market in over- the-counter drugs at pounds 6.5bn in 1992. The UK market was valued at pounds 550m.
SmithKline lodged its application for over-the-counter status for Tagamet in the US as long ago as 1989 and will have a meeting with the FDA advisory committee in September.
US approval is unlikely this year, although SmithKline is keen to win approval before Tagamet comes off US patent next May.
'Obviously the States is the real market to crack,' said Paul Woodhouse, an analyst at Smith New Court. 'These authorities tend to work independently, but if a European market gives it the thumbs-up they must think it bodes well.
'If Tagamet gets US approval such a product would generate several hundred million pounds' worth of revenue for the company.'
SmithKline believes Tagamet to be superior to existing over-the-counter antacids that neutralise acid in the stomach. Tagamet, whose active ingredient is cimetidine, reduces the amount of acid secreted in the first place, allowing the effect to last longer.
Zantac provided nearly half of Glaxo's pounds 1.4bn pre-tax profit for the year ended June 1992.