Eight former U.S. Treasury chiefs say leaving the European Union would be risky for Britain and threaten London's pre-eminence as a financial center.

The group, including both Democrats and Republicans, warn in an open letter to the Times of London on Wednesday that Britain "should not take for granted its global primacy when it is no longer the gateway to Europe."

The signatories are George P. Shultz, W. Michael Blumenthal, Robert E. Rubin, Lawrence H. Summers, Paul H. O'Neill, John W. Snow, Henry M. Paulson, Jr., and Timothy F. Geithner.

Former U.K. Defense Secretary Liam Fox, who is campaigning for Britain to leave the EU, dismissed the intervention as misguided. British voters will decide the issue in a June 23 referendum.