Superannuation funds with 70 per cent or more of their members in pension phase will lose franking credits under Labor's dividend imputation plan, according to Plato Investment Management.

Plato managing director Don Hamson, who has launched an online petition and attracted 500 signatures, said although some industry super funds had confirmed their members would be unaffected, retail funds with membership skewed to older age groups would be worse off, as would many self-managed superannuation funds.