Labour has demanded the immediate publication of an internal Downing Street report into allegations against Northern Ireland Secretary Theresa Villiers.
David Cameron confirmed that he has seen the results of an inquiry conducted by Cabinet Secretary Sir Jeremy Heywood into claims that Ms Villiers held an undeclared meeting while a transport minister with a lobbyist working for a company seeking to build a railway depot in the green belt.
St Albans MP Anne Main lodged a formal complaint with Sir Jeremy earlier this year accusing Ms Villiers of a potential breach of the ministerial code.
At Prime Minister's Questions in the House of Commons, Ms Main asked the PM: "Could you help in getting justice for my constituents, who want to know why an investigation into the meetings had by Theresa Villiers, the transport (minister), have not been reported on, despite four months of waiting and assurances that I would have the answer?"
Mr Cameron told the Conservative MP she would get an answer "shortly", but he was pressed again by Labour's Luciana Berger, who asked him to ensure that Sir Jeremy responds before the House rises for its Christmas recess on December 19.
The PM replied: "I have seen a copy of the Cabinet Secretary's response and I am confident that it will be sent in the next few days."
Following the exchange, a senior Number 10 source was asked whether Mr Cameron continued to have full confidence in Ms Villiers, having seen a copy of the report. He replied: "Yes. She is doing a very good job in Northern Ireland."
Asked whether the report would be released before the Christmas break, the source replied: "Let's just wait and see what Sir Jeremy Heywood comes back with."
But a Labour spokesman said: "These are serious allegations of possible breaches of the ministerial code by Theresa Villiers when she was minister for transport.
"We have already called for the Prime Minister to confirm that the Cabinet Secretary is investigating, which we now know.
"We want the findings of that investigation published as soon as possible. Given the that Prime Minister has confirmed that he has the findings, we believe they should be published immediately."
The Sunday Times reported earlier this month that Ms Villiers had a private lunch with a representative of a company campaigning for planning permission for a £400 million international rail freight exchange on 300 acres of green belt between St Albans and London.
The proposal for five warehouses served by railway sidings, first lodged by developers Helioslough in 2006, has faced considerable opposition from people living in the village of Radlett and has previously been blocked - most recently by Communities Secretary Eric Pickles in 2010 - because of fears of damage to the green belt.
However, Helioslough later won a judicial review into his decision and the proposal is once again being considered by the Government.
The Sunday Times reported that, following freedom of information requests from Ms Main, Villiers confirmed she had had lunch "in a private capacity" in August 2011 with Simon Hoare, who is chief lobbyist for Helioslough and a Tory councillor in Mr Cameron's Oxfordshire constituency . She had allegedly failed to declare the lunch on the official list of ministerial meetings.