Apple would have paid a tax rate of only about 15 per cent last year, far below the 25.2 per cent it reported, had it not used a form of reserve accounting that sets it apart from other big US technology companies.

The rare accounting treatment has helped to distract attention from Apple at a time when the tax-avoidance strategies of other cash-rich US tech companies, notably Google, have come under public attack, according to tax experts.

However, Apple’s tax planning is likely to come under the microscope tomorrow when Tim Cook, chief executive, appears before the US Senate’s permanent investigations subcommittee. The committee, which has already grilled executives from Microsoft and Hewlett-Packard, has targeted the techniques US companies use to shift profits to lower-tax countries.