Tom Bergin

Four of the biggest US technology groups collectively hold an estimated $124 billion in US Treasury debt, much of it offshore, earning them tax-free interest, the UK's Bureau of Investigative Journalism (BIJ) said.

The finding means Apple, Microsoft, Google and Cisco hold a large proportion of the $254.9 billion held in their foreign subsidiaries in US Treasuries, according to securities filings. Bringing the money home would trigger a tax bill, so the companies keep it offshore, partly to fund foreign expansion but also, executives say, to avoid the tax hit.

Sen Carl Levin (D-MI), who has campaigned for years against tax avoidance, was quoted saying that if US corporations invested offshore funds in US government debt, this income should face US taxes.

"Those funds ought to be treated as having been repatriated and subject to US tax," Sen Levin said.