BT could face strikes following a dispute over its proposed pay awards for staff.
The Communication Workers Union (CWU) has now threatened to ballot 50,000 employees unless the telecommunication giant withdraws it plans.
In a letter to members Andy Kerr, deputy general secretary of the CWU said BT's position was "not acceptable" and negotiations had reached a "critical stage", the Mirror reported.
The letter said the CWU was demanding a 5% increase in pensionable pay, but BT had offered 2%, which the union claimed would amount to a cut in real terms as inflation was more than 3%. The increase would also not contribute towards staff pensions.
Kerr said: "If BT is unwilling to make an offer that fairly reflects your contribution to the company's success, and the rise in the cost of living, the CWU will have no alternative but to initiate a ballot for industrial action."