British Aerospace (BAe) is demanding that firms prove a commitment to pro bono work, or risk being kicked off its panel.
The blue chip client's call comes as companies increasingly review law firms ethical behaviour.
BAe legal director Stuart Caroll has written to the company's panel firms asking them what pro bono work they are doing, and calling on them to support the Solicitors Pro Bono Group (SPBG).
The company is now considering making a commitment to pro bono work a requirement for panel firms.
It is believed to be the first public ethical initiative, although a source at one of BAe's panel firms, Eversheds, says: "Three or four clients have recently let it be known that a commitment to pro bono must be part of any tender."
SPBG is lobbying the Government to introduce a question on commitment to pro bono into government tender documents.
Terence Black, deputy legal director, finance, says: "British Aerospace has a vision. We have five corporate values and one of those is partnership with the community. We want our law firms to be aligned to our values, to have a similar culture to our culture.
"Our view would be that we would think very hard before instructing a firm that said it wasn't interested in pro bono work."