BT generated £623m for the Hampshire economy last year, according to a report.
The study - by DTZ Consulting - also shows that the communications company supports 12,300 jobs in the county through direct employment, expenditure with contractors and suppliers and the spending of its employees.
The overall beneficial financial impact of BT activities is expressed as a 'Gross Value Added' contribution.
For the entire south east, BT's GVA totalled 3.5bn - two per cent of the region's total.
John Weaver, BT's regional director for the south east, said: 'BT contributes some very big numbers to the south east's bottom line.
'These latest figures confirm the unique and vital role the company plays in the prosperity and well-being of the region.
'Every local authority area and every part of society benefit from our day-to-day activities and our commitment to be at the heart of every community. BT people across the south east have a genuine desire to make a positive contribution to their society.
'It is vital as a business that we continue to grow and help our customers, stakeholders and employees to thrive as we work together to create a more prosperous south east.'
The report highlights that BT in the south east is responsible for the employment of more than 10,530 people - 10,265 direct employees and 265 contractors - with a total income of 366m.
It also provides work for a further 56,000 people through the company's expenditure with businesses supplying its equipment and services and the spending of its workers. It also spent 2.77 billion with suppliers last year.