Shadow leader of the Commons Theresa May has demanded a ruling on whether Culture Secretary Tessa Jowell has breached the ministerial conduct code.
Tessa Jowell co-owns a £700,000 home with David Mills
Ms May has written to Cabinet Secretary Sir Gus O'Donnell asking him to make a ruling on the matter.
Tessa Jowell is facing questions about the business dealings of her husband, David Mills. She denies any wrongdoing.
International lawyer Mr Mills denies taking a £344,000 bribe from Italy's Prime Minister Silvio Berlusconi.
The Sunday Times says it has identified a link between a loan secured on a house jointly-owned by the couple and money which Italian prosecutors allege was a bribe.
Ms Jowell co-signed a mortgage on the £700,000 home in Kentish Town, north London, which she jointly owns with Mr Mills, but said the deal did not mean a conflict of interest.
"I agreed that we would take out a loan on our house. That is not unusual, it's not improper, and it's certainly not illegal," she told reporters outside her home on Sunday.
Labour colleagues have been quick to jump to Ms Jowell's defence, with the Lord Chancellor, Lord Falconer, saying she has his full support.
Speaking on BBC's Politics Show Lord Falconer said he was confident she had adhered to the ministerial code of conduct.
"She's been a really good minister at every level. She should stay in her job," he said.
Italian officials say Mr Mills, 61, was paid to give false evidence in court for Mr Berlusconi.
Mr Mills admits writing a letter to his accountants where he appears to be describing a payment, but claims he was inventing a scenario to get tax advice.
In a statement, Ms Jowell said: "I signed a charge over our jointly-owned home to support a loan made to my husband alone by his bank.
"I am satisfied that no conflict of interest arose out of this transaction in relation to my ministerial duties.
"As is standard practice in relation to legal proceedings, it would be inappropriate for me to comment further."
But Ms May said it was not essential to know about the Italian investigation into Mr Mills.
She said she was concentrating on whether the ministerial code had been breached by Ms Jowell.
Ms May wants to know if "the ministerial code been adhered to"
"I don't know what she knew about the financial, those financial transactions," she said.
"But all I'm asking is, has the ministerial code been adhered to, particularly obviously in relation to any requirement to notify in relation to financial transactions."
Meanwhile, former Cabinet minister David Blunkett said he hoped the Cabinet Secretary would move quickly to reassure people that Ms Jowell had not breached the ministerial code of conduct.
He said he did not believe there was any evidence "whatsoever that Tessa Jowell's public role as a minister, past or present, has anything whatsoever to do with her husband's affairs".
And Minister for Communities, David Miliband, told Sky News that "we should let the due process go through" in Italy, where, he said, there was a lot of "ding dong" because of elections there.