The shift in the "atmosphere around tax" globally is "significant" and has "hardened," the chief executive of WPP, which famously moved to Ireland to avoid U.K. tax regime change, has told CNBC.
"Tax regimes are being tightened up," Sir Martin Sorrell said, and cited a clampdown on "postbox companies."
There has been increased international focus on tax avoidance and tax havens since the financial crisis, with the Organisation for Economic Co-operation and Development recently focusing on closing down tax loopholes. These could include the so-called "Double Irish Dutch," where companies use Irish and Dutch subsidiaries to switch profits to lower tax jurisdictions.
Read MoreOECD pushes for tougher tax reforms