The 50 largest companies in the United States have now shifted $1.6 trillion of their profits to offshore tax havens to reduce their tax burden, according to new analysis.

The 50 largest US companies, including Apple, Microsoft and Pfizer, parked $200 billion in 2015 alone, according to Oxfam America, a charitable organization affiliated with the UK-based Oxfam.

$200 billion of their profits to offshore tax havens in 2015 alone, taking the total to approximately $1.6 trillion, according to new analysis.

Although not illegal, the companies "used a secretive network of 1,751 subsidiaries in tax havens to stash" their earnings outside the US, the report said ahead of next week's meetings of the International Monetary Fund and World Bank in Washington, DC.

"Tax avoidance has become standard business practice across the globe. Corporate tax dodgers cheat America out of approximately $135 billion in unpaid tax revenues every year," Oxfam senior advisor Robbie Silverman said.

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Apple is at the top of the tax avoidance ranking with more than $200 billion in offshore accounts, followed closely by Pfizer with $194 billion and Microsoft with $124 billion, the report said.

The top rate of US corporate tax is 35 percent, one of the highest rates in the world. However, Oxfam said these 50 companies had an effective tax rate nearly 10 percentage points lower.

During his election campaign, US President Donald Trump proposed cutting the corporate tax rate to 15 percent, and allowing companies to repatriate their cash reserves with a one-time tax of 10 percent.

Oxfam criticized both proposals by Trump, saying the tax cut would benefit large profitable companies and wealthy shareholders at the expense of important anti-poverty programs.

"President Trump promised to fix the rigged political and economic system yet his tax reforms will further enrich powerful corporates at the expense of ordinary people and small businesses," Silverman said.

"The President and leaders in Congress must rethink their reforms and build a tax system that works for everyone and not just a fortunate few," he added.

A report released last year by Citizens for Tax Justice and the Institute on Taxation and Economic Policy found that Fortune 500 companies have managed to shelter $2.5 trillion in profits offshore to avoid paying about $717 billion in taxes to the US government.

The study showed that if all the Fortune 500 companies paid taxes on their sheltered profits, the US government would receive an astonishing $717.8 billion in revenue. To put that number in context, the federal budget deficit last year was $438 billion.