The anonymous four-paragraph letter, accompanied by a tube of unknown powder, was apparently posted on 20 February by someone who claims to be an employee of Wellcome.
It reads: "Dear Wellcome Trustee. We, soon to be the ex-employees of the Wellcome Foundation, wish to bring to your attention our extreme unhappiness at the role you and your fellow trustees have had in ruining our lives and careers.
"For you to appreciate the devastation we are undergoing now and in the future as over 80 per cent of us are made obsolete, we include a small reminder [the powder] of our ability to respond."
The penultimate paragraph contains a chilling warning. "We are normally a passive workforce and have never resorted to threat and force, but extreme measures demand extreme reprisals.
"Basically, as the takeover by Glaxo proceeds and as the Wellcome workforce is disposed of, we shall use the means of chemical, biological, radiological and any other measures as required to effect [sic] the lives of yourself, your families, your employees and their workplaces."
It continues: "To minimise the effect on innocent employees we will use the following call-sign to signify the genuineness of the message: Happy Dreams."
One of the trustees, who declined to be named because of concerns for his family, said: "I know about it [the letter]. It's bloody awful. I can't say more because the police are involved." He confirmed a trustee had received a tube of powder.