Apple CEO Tim Cook has denounced the European Commission's decision that Apple's tax benefits in Ireland have been illegal as an "unprecedented" move with "serious, wide-reaching implications".

In a letter to customers this afternoon, Cook affirmed that the tech giant followed all Irish guidance when it came to taxes and has paid all the tax that it owes.

He said that the EU verdict was "an effort to rewrite Apple's history in Europe, ignore Ireland's tax laws and upend the international tax system in the process."

This morning, European Commissioner for Competition Margrethe Vestager announced that Apple had received illegal Irish tax benefits of up to €13 billion.

Ireland must now recover the illegal aid, however both the Irish government and Apple are likely to fight the decision.

In his letter, Cook recalled the late Steve Jobs opening Apple's Cork factory in 1980, expressed his pride regarding the company's impact on the local economy in the decades since, and argued that the Cork base has helped to create and sustain over 1.5 million European jobs since.

In one particularly pointed paragraph, he wrote:

"The Commission’s move is unprecedented and it has serious, wide-reaching implications. It is effectively proposing to replace Irish tax laws with a view of what the Commission thinks the law should have been.

"This would strike a devastating blow to the sovereignty of EU member states over their own tax matters, and to the principle of certainty of law in Europe.

"Ireland has said they plan to appeal the Commission’s ruling and Apple will do the same. We are confident that the Commission’s order will be reversed."