US pharmaceutical giant Pfizer could make a hostile bid for AstraZeneca as early as today.
Its £63 billion takeover offer was rejected after the UK company said it "substantially" undervalued the business.
That has forced Pfizer to consider its next move, with industry experts believing a hostile bid - which would aim to bypass the AstraZeneca management and target shareholders directly - could come soon.
Last week, Pfizer insisted it was committed to building a research and development hub in Cambridge which was planned by AstraZeneca.
The £300m base at the Cambridge Biomedial Campus would house about 2,000 researchers.
But a hostile bid would trigger increased calls for the offer to be referred to the competition authorities and would also heighten union fears over jobs if the takeover was successful.
The GMB union said that the takeover approach of AstraZeneca by US drugs giant Pfizer has created added insecurity for workers and that any bid should be referred to competition authorities.
It called for a "bankable commitment that manufacturing and R&D [research and development] will continue here" amid fears these could be exported to the Far East.
GMB national officer Allan Black said the Government had a duty to protect against a "hostile approach" which would see business going abroad, adding that such a move would also weigh on the balance of payments deficit.
AstraZeneca shares climbed more than 15% to over 4700p as trading opened on London today, representing an increase in value of nearly a quarter, or around £11 billion, since before the takeover speculation began last week.
It announced last year that it was building a new £330 million research and development site on Cambridge Biomedical Campus by 2016, employing about 2,000 staff, with corporate staff also moving to the city from London.
The company said the plans would boost the chances of Britain discovering the next generation of medicines and that it would become a "global leader in biopharmaceutical innovation".
Chief executive Pascal Soriot said the move confirmed Astra's "long-standing commitment to the UK".
Astra also last year announced it would end research and development at Alderley Park in Cheshire, with about 1,600 roles moving over three years. Combined with the closure of its London headquarters, it would mean 700 UK jobs cut by 2016.