BT's 80,000 staff will be told company bosses and union leaders back remaining in a reformed EU, according to reports.
In an email to workers to be sent on Monday, the telecoms giant's chairman, chief executive and leaders of the Prospect and CWU unions will urge employees to vote - but not tell them how, the BBC reports.
It will reportedly refer to independent research that suggests the economy will suffer as a result of a Brexit, however.
Leave campaigners dismissed the letter and branded the notion of a reformed Europe as "meaningless and hollow".
Matthew Elliott, chief executive of Vote Leave, said: "Everyone knows that renegotiation was an utter failure that achieved nothing.
"If we really want to take back control of our economy, our democracy and our borders then we have to Vote Leave."