Pension fund investors are moving to put pressure on American lawmakers in a push for legislative action to halt companies inverting their place of incorporation offshore.

The California Public Employees' Retirement System (CALPERS) had in recent weeks called on several companies that had inverted to shift their corporate addresses back to the US - or face the threat of the pension fund pulling its investment in the company's stock.

And now CALPERS shareholders have stepped up their campaign against corporate inversion companies - which move their place of incorporation to an offshore jurisdiction, such as Bermuda, in a bid to avoid paying US taxes on foreign income - with a letter campaign to encourage legislators to continue drafting legislation to penalise companies that move offshore.

The heated corporate inversion debate - which has centred around a wave of companies moving their place of incorporation offshore for tax reasons, including to Bermuda - prompted a flurry of draft legislation being filed throughout the year in the US Congress.

None of the bills got passed into law during the session but predictions are that the anti-inversion lobby has too many supporters for the issue to just die away.

The Dow Jones News service yesterday reported that CALPERS chief investment officer Mark Anson, in a draft letter set to be reviewed by the giant pension fund's investment committee Monday, asked key lawmakers for their commitment "to close the expatriate loophole and stop corporations from fleeing to tax haven countries".

The news service said this development was "the latest sign that despite a lack of congressional action this past year on so-called corporate inversions, investors aren't willing to let the issue die".

Mr. Anson continued in his letter: "As part of this effort to halt further expatriation, we urge Congress to take immediate action to eliminate the tax incentives that entice companies to relocate to tax haven foreign nations."

And Mr. Anson voiced concerns over limits on shareholders being able to take corporate wrong-doers to court if a company incorporated outside the US.

"Tax benefits that encourage companies to relocate overseas hinder shareholders' ability to pursue their rightful legal remedies in the event of mismanagement or illegal actions," he wrote.

The Dow Jones said the letter was to be sent to nine lawmakers, including two who have been behind legislation to crack down on corporate inversions - Rep. Bill Thomas, chairman of the House Ways and Means Committee; and Sen. Charles Grassley, soon-to-be Senate Finance Committee chairman.