Sidley Austin Brown & Wood raked in $15m (£8.72m) for issuing “cookie cutter” opinion letters, backing the use of potentially illegal tax shelters, it emerged during the top 10 US firm’s bruising testimony before the Senate last month.

Giving evidence before the US Senate hearings, legacy Brown & Wood managing partner Thomas Smith admitted that the firm charged around $50,000 (£29,100) for every one of the “mass-produced” letters it sent out to more than 300 clients.