BT chairman Sir Mike Rake and chief executive Gavin Patterson teamed with the Communications Workers Union, and Prospect to issue the note, which sought to highlight their concerns, without explicitly encouraging staff to vote Remain in the upcoming referendum.
However, the move has been attacked by former trade secretary Peter Lilley, who accused the pair and seeking EU protection against the break up of its business.
Ofcom controversially ruled against separating BT and its infrastructure unit Openreach in February.
Read More: BT avoids Openreach spin-off - for now
And Lilley has now claimed that the watchdog’s decision was motivated by fear of a challenge from European courts.
“Apparently OfCom would have gone further in this direction but for fear that they would be challenged in the EU Court on the basis of the EU Framework Directive.
“This is another example of big business bosses relying on the EU to protect their cosy practices – and another reason why British business as a whole will benefit if we take back control,” Lilley said.
The former minister added that BT would be among the businesses least affected by a Brexit vote and accused the BT bosses of abusing their position.
A BT spokeswoman responded: "BT earns about a fifth of its revenues outside the UK, selling comms and IT services to large companies and other organisations through Europe and beyond.
"EU initiatives helping us to rent space on other national networks have been an important factor in our success. We want that open access and success replicated in other markets, such as the USA, where having the EU backing our campaign for change is of immense value.”