SmithKline Beecham and Glaxo Wellcome announced on Friday that they were discussing a merger that would create the world's eighth largest company on market value, and the UK's biggest company, ahead of HSBC Holdings and Lloyds TSB Group.
Both drugs giants have described the claims as "pure speculation", but the Manufacturing, Science and Finance Union (MSF) said staff at both companies were "totally shell-shocked" at the news.
MSF general secretary Roger Lyons said: "There are around 20,000 people employed by both companies in the UK, and we believe up to half those jobs could be at risk because the two companies will duplicate each other."
Mr Lyons added: "No-one knows anything at all. It's all been done in secret by a handful of men in suits. People are confused. We are demanding urgent talks to find out what on earth is going on."
The union is organising emergency meetings with the companies' 21,000- strong UK workforce to discuss the implications of a merger.
Mr Lyons has sent letters to the chief executives of both companies and to Margaret Beckett, President of the Board of Trade, demanding urgent talks. In his letter, Mr Lyons said: "The two companies are not basket cases. They don't need rescue. The case for merger has not been made."
The companies' UK sites include Brentford, west London, Harlow in Essex, Irvine in Scotland, Stevenage in Hertfordshire and Dartford in Kent.
Glaxo spokeswoman Nancy Pekarek said she could not comment on the union's claims, and no further comment would be made while discussions were still taking place. Lynne Smith, of SmithKline Beecham, said talk of a merger was "pure speculation".
SmithKline Beecham and Glaxo Wellcome would, if merged, have a worldwide value exceeding pounds 100bn, say stock market analysts. Together they would control 7.5 per cent of the global pharmaceuticals market.Reuse content