"Many Wellcome employees have given decades of their lives, skills, expertise and commitment to Wellcome, only to find themselves facing the prospect of redundancy."
The letter says that Sir Henry Wellcome, who founded the trust, had a strong commitment to the wellbeing of Wellcome employees.
MSF, which is also the largest union at Glaxo, asks the trust to consider the impact on staff of allowing either Glaxo's bid for Wellcome or an offer from any rival to go ahead.
Union sources said staff believed the higher bid from a white knight being sought by Wellcome would also trigger large-scale redundancies. This view is shared by many investment analysts.
The trust has committed its 39.5 per cent stake in Wellcome to Glaxo unless a higher offer emerges.
Meanwhile, Glaxo's financial adviser, Lazards, confirmed it had sent a "detailed questionnaire" to Wellcome's adviser, Barings, in the past few days asking for information on a wide range of financial and commercial issues.
Under the rules of the Takeover Panel, a defending company is normally only required to give "equality of information" to competing bidders if it makes a public announcement of the existence of an alternative potential bidder. An announcement has been absent in Wellcome's case despite its high-profile search for a preferred suitor.
A source close to Glaxo said: "It depends on how you define potential bidder. There are unusual circumstances in this bid because Wellcome has said it is making presentations to people who may be interested in making a bid."
In the continuing war of words, a Wellcome spokesman said it was aware of the Panel rules on disclosure of equal information, and that the questionnaire from Lazards showed the other camp increasingly believed a rival bid to Glaxo's £9bn-plus offer was possible.
Unconfirmed reports that Glaxo had approached Zeneca with a takeover offer shortly before launchings its bid for Wellcome "shows, if it is true, that Glaxo just want any deal to cover up the fact that it has gone ex-growth", the spokesman said.
Sandoz, the Swiss pharmaceuticals group, yesterday ruled itself out as a potential white knight for Wellcome.