Perth-based Peak Performance, which promotes the K2 scheme revealed in June to have been used by Carr and 1100 other clients to shelter £168 million of tax, is one of between 50 and 100 "active promoters" of avoidance schemes.
In each of the past four years, more than 100 new schemes have been disclosed to HM Revenue & Customs. It estimates there are 30,000 users of mass-marketed schemes led by "partnership loss" wheezes, where taxpayers invest in a partnership with "losses", and "employment intermediary" schemes which use agent companies to replace salary with supposed loans to minimise tax and national insurance.
There are also an estimated 3400 users of Employee Benefit Trust schemes, which received an unexpected boost this week when the Tax Tribunal ruled that much of the scheme adopted by Rangers between 2001 and 2010 was lawful.
HMRC has been successful in 85% of legal challenges to schemes, but the report warned: "The long time-scale for resolving cases increases the economic advantage to taxpayers of using an avoidance scheme because, however the case is resolved, taxpayers receive the cash-flow advantage of not paying tax while the investigation is under way.
"If the case is settled in HMRC's favour, then interest is charged from the date the tax was due. The main economic factors that might deter a taxpayer from buying avoidance schemes are the fees, and the burden of an HMRC investigation."