The European Commission will today publish its reasons for opening an in-depth inquiry into the Irish government’s tax treatment of Apple, a Commission spokesman said.

The European Union’s competition watchdog is looking at whether a number of countries’ benign tax regimes for multinational companies, which helps to attract investment and jobs that might otherwise go elsewhere, represents unfair state aid.

A US Senate investigation into Apple’s tax affairs in 2013 showed how Apple had used Irish-registered companies that were tax resident in no country to shelter tens of billions of dollars in profit from tax. Theoretically, if the Commission ruled the tax treatment constituted state aid, Apple could be forced to repay billions of dollars in tax savings.

However, some tax lawyers said they doubted the Commission could enforce such a ruling and that it is more likely Ireland would simply be forced to change its light-touch approach to taxing multinationals, which other European countries say robs them of tax revenues.

Apple has denied receiving any selective tax treatment and the Irish government reiterated that it is confident it had not breached state aid rules.