David Blunkett last night faced calls for an independent inquiry into his relationship with a DNA-testing firm after disclosing that he had bought shares in the company in trust for his sons.

Mr Blunkett said there was no conflict of interest between the shareholding and his official duties as Work and Pensions Secretary.

But Shadow Leader of the Commons Chris Grayling said his comments left important questions unanswered and called for an independent inquiry to clear up the matter.

In a letter to Mr Grayling, Mr Blunkett said he had bought a three per cent shareholding in DNA Bioscience - the trading name of DNA Structures Ltd - which was held in trust for his three elder sons.

He said he had informed both the Permanent Secretary at the Department of Work and Pensions (DWP) and the Registrar of Members' Interests of the purchase.

He stressed that neither the DWP nor the Child Support Agency (CSA) - for which he has ministerial responsibility - had any contracts with DNA Bioscience.

He said that if the company was ever to seek any contracts with the DWP or the CSA, he would not take any part in the decisions.

''There is no conflict of interest between my official duties and DNA Bioscience. Neither the department nor the Child Support Agency has any contracts with DNA Bioscience,'' he said in his letter to Mr Grayling.

''The contract for the CSA paternity tests is not due for renewal until 2008. I have never made representations to the Department or the Child Support Agency (or any other Government department) on behalf of DNA Bioscience."

Mr Blunkett briefly became a director of DNA Bioscience for two weeks before the General Election but resigned after he was reappointed to the Cabinet.

Mr Grayling said Mr Blunkett still had not answered questions about reports that he continued to meet businessman Tariq Siddiqi whose family owns much of the company.

He said Mr Blunkett had also failed to deny that discussions were taking place between the DWP and DNA Bioscience over possible future business. ''This matter has become extremely serious and needs to be properly investigated,'' Mr Grayling said.