Kenya's opposition coalition has urged the president to dissolve his cabinet following new allegations of corruption against four senior ministers.
John Githongo resigned after receiving death threats
Over the weekend, a report by former anti-corruption chief John Githongo was published, saying the four had tried to block corruption investigations.
Two of the four, who include the vice-president and the finance minister, publicly denied the charges.
President Mwai Kibaki was elected in 2002 on a pledge to fight corruption.
But Western diplomats have launched strongly-worded attacks on his administration, saying corruption has continued unabated.
Energy Minister Kiraitu Murungi has angrily denied the claims, which he said were "untrue" and an attempt to bring down the government.
Mr Githongo resigned a year ago, amid reports that his life had been threatened because of his investigations into corruption allegations.
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."
In a 31-page report seen by the BBC, Mr Githongo accused Mr Murungi, Vice-President Moody Awori, Finance Minister David Mwiraria and sacked Transport Minister Chris Murungaru of having links to 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 said he was told by ministers that his "digging around had gone too far" and that he should "go easy" with his investigations.
President Kibaki is under increasing pressure
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.
He said President Kibaki had known about the scam but had done nothing to stop it.
Mr Awori has also denied the claims, according to local media.
Mr Murungaru was internal security minister at the time of the Anglo-Leasing contract, responsible for forensic laboratories.
He was sacked from government after the UK and US refused to give him a visa.
He has denied all wrongdoing.
The government promised to investigate the latest claims but said there would be no "lynch-mob tactics".
Mr Kibaki is already under pressure after losing a referendum last year on a new constitution.
Kenya has been plagued by corruption for many years.
Last year, the Anti-Corruption Commission said that 80% of new police recruits had either paid bribes or used their connections to get jobs.