A complaint about how David Blunkett gave a first class rail ticket to his then lover Kimberly Quinn has been put to Parliament's standards watchdog.
Mr Blunkett denies the visa allegation
Sir Philip Mawer is looking at whether there is evidence for an inquiry after a member of the public complained.
Ex-civil servant Sir Alan Budd is already looking at claims the home secretary "fast tracked" a visa application for Mrs Quinn's nanny.
No 10 has denied that Tony Blair has prejudged the outcome of that probe.
Mrs Quinn, who is seven months pregnant, was admitted to hospital on Monday night amid the stress caused by the controversy.
Her husband, Stephen, said the problems were not serious but he did not know when she would be discharged.
The home secretary has confirmed giving Mrs Quinn a train ticket assigned to him.
He says he believed he was entitled to claim for a partner's travel, but has offered to refund the money if necessary.
In the separate independent inquiry set up by the government, Sir Alan Budd is examining whether Mr Blunkett intervened in the visa application.
The Tories said the inquiry should have been headed by a judge and look at all the allegations.
And Sir Alistair Graham, chairman of the Committee on Standards in Public Life, criticised Mr Blair's prediction that Mr Blunkett would be exonerated.
He told BBC2's Newsnight: "What the prime minister has to remember is that he is the final arbiter, the final person who will decide, having seen the results of the inquiry, what action is appropriate."
The prime minister's official spokesman denied the probe's remit was too narrow, saying other, less serious, allegations would be looked at by the Home Office.
He said that by giving Mr Blunkett his public backing, Mr Blair had been expressing trust and confidence in his home secretary - and not pre-judging the review.
Mr Blunkett pictured with Mrs Quinn at a function last year
Mr Blair intends to review the code that governs the conduct of ministers after the next general election but the government says that decision was taken some time ago and was not prompted by the Blunkett claims.
Mr Blunkett requested an independent review into claims he fast-tracked a visa application for permanent residency by Leoncia Casalme, who left Mrs Quinn's employment two months ago.
He is said to have checked the application was filled in correctly but not intervened to get it approved.
The visa claim was among six allegations made in the Sunday Telegraph newspaper, including details of the rail ticket for Mrs Quinn.
It said Mr Blunkett, 57, shared confidential security information with Mrs Quinn, publisher of the Spectator magazine.
This included telling her parents to avoid Newark Airport near New York hours before a security scare.
It is also alleged that Mr Blunkett took her to Spain accompanied by bodyguards at taxpayers' expense.
Another claim is that he ordered his government chauffeur to drive Mrs Quinn between London and his Derbyshire home.
Requesting the inquiry, Mr Blunkett said he regretted the time and resources being used but they were necessary to lay the accusation to rest.
Of the other allegations, Mr Blunkett's spokesman said information about the New York security scare was already in the public domain.
And he said his bodyguards were already in Spain as part of his normal protection.
Mr Blunkett said Mrs Quinn had only been given lifts on trips his chauffeur was already scheduled to make on government business.
He also dismissed as "nonsense" claims that he had asked for a policeman to be posted outside Mrs Quinn's central London home during a May Day riot.