Kenya's government has invited former anti-corruption czar John Githongo to return home and has guaranteed his safety following death threats.
John Githongo resigned after receiving death threats
Mr Githongo was urged to end his exile in the UK and defend his latest charges of corruption by top ministers.
The allegations have threatened to bring down the government.
Civil rights groups, church leaders and lawyers have urged President Mwai Kibaki to suspend the ministers until investigations are carried out.
In the first government response to the allegations, Lands Minister Amos Kimunya said such calls were "premature", as investigations were under way.
He suggested that Mr Githongo was straying beyond his brief with his 31-page report into the Anglo-Leasing scandal, in which a huge contract given to a non-existent company, Anglo-Leasing, to print new high-technology passports, and build navy ships and forensic laboratories.
"He is an advisor, or was an advisor to the president, on matters to do with corruption," Mr Kimunya said.
"He was not an investigator into corruption and his presence or absence has not in any way stopped the investigations on corruption in this country."
On Wednesday, the World Bank urged Kenya's president to take tough action against any cabinet ministers found to be corrupt.
The warning came as the World Bank approved a new $25m loan to help fight corruption - a decision slammed by former UK Kenya envoy Sir Edward Clay.
Sir Edward, who has condemned Kenya for not tackling graft, said the new loan would feed the "pig of corruption".
"The Anglo-Leasing cases represent an excellent opportunity for the authorities to invoke the disciplinary provisions of the code of conduct signed by the new cabinet weeks ago," said World Bank Kenya director Colin Bruce.
"I believe that this is an historic moment for the government to signal where it stands on the issue of political accountability," he said.
President Kibaki is under increasing pressure over corruption
President Kibaki was elected in 2002 on a pledge to fight corruption.
Some donors, including the UK, have suspended some aid to Kenya over concerns about corruption and Sir Edward, who retired last year, thought the World Bank should have sent out a tough message.
"How can the World Bank be so insensitive and hapless to announce new loans to Kenya?" reports the Guardian newspaper.
"They have added insult to injury by feeding the pig of corruption in Kenya when many Kenyans were beginning to hope they might smell the bacon beginning to fry."
Over the weekend, Mr Githongo's leaked report said his attempts to investigate the Anglo-Leasing scandal were blocked by four top ministers - Vice-President Moody Awori, Energy Minister Kiraitu Murungi, Finance Minister David Mwiraria and sacked Transport Minister Chris Murungaru.
Mr Murungi and Mr Awori have publicly denied the claims.
Mr Murungi said the report was "untrue" and an attempt to bring down the government.
Mr Githongo resigned last year amid reports that his life had been threatened.
The money raised by the alleged scam was to be used to fund the ruling Narc coalition's campaign in elections due next year, Mr Githongo said.
Following the leaking of the 31-page report, the opposition has urged President Kibaki to dissolve cabinet.
Opposition Orange Democratic Movement leader Uhuru Kenyatta said: "This is clear evidence that the government can no longer be trusted to conduct detailed and honest investigations into this saga."