BAE Systems' £20m of payments to a company linked to John Bredenkamp, a Zimbabwean arms dealer, are part of a web of opaque international dealings that the company has proved reluctant to explain despite launching a high-profile inquiry into its business ethics.

The latest payments highlight the contentious role of Red Diamond Trading, a secretive BAE offshore subsidiary that was dissolved last year at a time when the company was making a big effort to improve its damaged reputation.