William Hague has warned Tony Blair that "vital issues" surrounding Peter Mandelson's resignation and the Hinduja passport affair remain unresolved.
The Tory leader said his party will press for full disclosure of events after Srichand Hinduja was granted naturalisation when Mr Mandelson's intervened.
A raft of parliamentary questions are being tabled, asking whether any other ministers were involved in the Hinduja case and whether and why Minister for Europe Keith Vaz wrote to both Mr Blair and Mr Mandelson about it in 1997.
Downing Street has confirmed the inquiry into Mr Hinduja's application would cover Mr Vaz's representations to the Home Office.
"There remain many vital unresolved issues surrounding the Hinduja passport affair," Mr Hague told a parliamentary press gallery lunch.
He said the "whiff of suspicion" was hanging over the Government.
Mr Blair has attempted to draw a line under Mr Mandelson's resignation. He spent only a few seconds referring to the incident at the weekly Cabinet meeting in No 10.
The premier's official spokesman sought to bring the episode to a close, saying: "Yesterday was yesterday, today is today."
The Prime Minister this afternoon visited the North East without speaking today to Mr Mandelson, a long-standing confidant and ally.
Mr Blair's spokesman also stressed Mr Mandelson, who had been due to head Labour's election campaign alongside Chancellor Gordon Brown, would now play a minor, if any, role.