BT bosses and union leaders have sent a joint letter to staff saying they want the UK to stay in a reformed EU.
The firm's chairman and chief executive and the leaders of the CWU and Prospect unions, said the vote would have a big impact on the economy and company.
They said it was up to staff how they voted in the EU referendum but urged them to cast their ballot.
Vote Leave said talk of a "reformed Europe" was "meaningless and hollow".
In other referendum developments:
- Migration Watch, a think tank that wants reduced immigration, published a study forecasting net migration to the UK would run at more than 250,000 a year for at least 20 years if the UK stayed in the EU
- Former PM Gordon Brown will try to encourage Labour voters to back Remain by telling them they have most to gain from keeping Britain in the EU
- Treasury select committee chairman Conservative MP Andrew Tyrie is to announce he will back the Remain campaign, saying leaving the EU would almost certainly lead to a "short-term economic shock"
The BT letter said the referendum was so important that the signatories felt they had a duty to explain their views.
The signatories are BT chairman Sir Michael Rake, chief executive Gavin Patterson, Communications Workers Union (CWU) deputy general secretary Andy Kerr and Prospect national secretary Ben Marshall, which have members among BT's 81,400 staff.
Sir Michael Rake, said: "We have a very good work force, we have a lot of people working in our global services business who are international who come and work here in London.
"So this is a great benefit to the company, great benefit to the UK economy. It's not the only factor which our employees must weigh up. There are other factors that concern them, but we have an obligation to at least write.
"Our trade unions share that view and we just write in very factual terms around those issues."
The letter said there had already been a slowdown in investment and economic activity because of uncertainty surrounding the referendum.
It pointed out that most independent experts had said there would be an economic down-turn if Britain left the EU, lasting several years, and that it would effect BT, one of the UK's biggest companies.
It highlighted that about 20% of BT's sales were outside of the UK and that membership of the EU meant that import taxes and other trade barriers were kept to a minimum.
The EU also helped to protect workers' rights and prevented a "levelling down" of employment laws, it said.
The letter encouraged staff to vote in the referendum whatever their point of view.
Responding to news of the letter to BT staff, Matthew Elliott, the chief executive of Vote Leave, said: "Sir Mike Rake backed the euro and opposed a referendum as did Gavin Paterson. They never even wanted their employees to have a say on this issue.
"Likewise their talk of a reformed Europe is meaningless and hollow - 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."
Earlier this year Sir Michael was one of the 36 bosses of big UK companies who signed a letter the the Times newspaper saying an EU exit would deter investment.
The firms, all FTSE 100 firms, also included Marks and Spencer and Vodafone. At the time Leave campaigners pointed out that two-thirds of FTSE 100 firms, including Tesco and Sainsbury, did not back the letter.
Earlier this month JCB chairman Lord Bamford wrote to his company's 6,500 employees in the UK to explain why he favours a vote to leave the European Union.
And on Saturday entrepreneur Sir James Dyson told the Daily Telegraph Britain would gain more from leaving the EU than it would lose.