Blackstone Group LP, which is preparing to go public, said on Friday that proposed U.S. Senate legislation on the taxation of private equity firms could materially increase its tax liability and lower the partnership's value.

Montana Sen. Max Baucus, a Democrat who chairs the Senate Finance Committee, and Iowa Sen. Charles Grassley, the ranking Republican, on Thursday introduced a bill to tax private equity firms like corporations, more than doubling their tax rate.

In an amended prospectus filed with the U.S. Securities and Exchange Commission, Blackstone said the proposed law would apply to it beginning with the 2013 tax year.

Blackstone said the legislation, if enacted, would cause a material increase in our tax liability when such legislation begins to apply to us. It also said changes that precluded it from qualifying under publicly traded partnership rules might result in a reduction in the value of our common units.

Blackstone is expected to conduct an initial public offering in the last week of June, raising an estimated $3.87 billion to $4.13 billion.