Karl Rove, the former trusted adviser to President George Bush, will be compelled to testify on Capitol Hill in a high-profile investigation of alleged political meddling by the White House in decisions made by the US Justice Department.

John Conyers, the chairman of the House judiciary committee, yesterday took the unusual step of issuing a subpoena compelling Mr Rove to testify in a case that threatens to disgrace the Bush government. He had tried in vain for a year to persuade Mr Rove to come forward voluntarily.

The committee is looking into what role the White House may have played in the sacking of nine US attorneys during 2006 Ð an affair which eventually led to the resignation of Mr Bush's last attorney general, Alberto Gonzalez. Also under scrutiny are the circumstances of the prosecution and subsequent imprisonment of the former governor of Alabama, Don Siegelman.

"This will make Watergate look like child's play when it is fully investigated," Mr Siegelman told an Alabama newspaper last week, referring to his case and the dismissal of the prosecutors. He has repeatedly insisted that the White House, and Mr Rove in particular, was behind his legal woes.

Mr Conyers said he had no choice but to order Mr Rove to appear on Capitol Hill and testify under oath on 10 July. The scandal could prove embarrassing for media organisations who have recently hired Mr Rove as a political commentator, including Fox News and Newsweek.

"It is unfortunate that Mr Rove has failed to co-operate with our requests," Mr Conyers said. "Although he does not seem the least bit hesitant to discuss these very issues weekly on cable television and in the print news media, Mr Rove and his attorney have apparently concluded that a public hearing room would not be appropriate. Unfortunately, I have no choice today but to compel his testimony on these very important matters."