Jack Straw, the Home Secretary, also disclosed that a warning that Saddam Hussein could be planning to flood the West with anthrax hidden in bottles disguised as duty-free goods was received 13 days before the Government alerted the ports.
In a further development, an Iraqi scientist who may have tipped off Western intelligence agencies about the anthrax threat and who was planning to defect is reported to have been arrested and thrown into prison.
MI5 alerted Customs and Excise officers and Special Branch police at British ports on a number of occasions this year following intelligence suggesting Iraq may be planning biological or chemical attacks.
While the most recent report told ports to be on the look out for anthrax, information about other named substances has been released. Iraq is known to have reserves of Agent VX, a highly toxic nerve agent used against the Kurds, mustard gas, which cause terrible skin damage, and sarin, the nerve agent used in Nazi gas chambers, aflatoxin, which destroys the immune system, and botulinum toxin.
A security source said the anthrax alert was "one of a number of warnings about the possible threat from Iraqi biological and chemical weapons" that had been issued to the authorities guarding ports and airports, including the Channel Tunnel and Eurostar.
The source stressed that there was no intelligence to suggest that Britain was to be specifically targeted. The anthrax alert said the threat was against "the enemies of Iraq".
Meanwhile, an official with UNSCOM, the UN body charged with rooting out Iraq's clandestine weapons programmes, confirmed yesterday that it was informed of the arrest of Nassir al-Hindawi by the Iraqi government earlier this month.
According to Baghdad, Mr Hindawi was found to be preparing his own flight from Iraq and had important documents about the biological weapons programme, which he intended handing over to western investigators.
Mr Hindawi is considered to have been one of the key pioneers of Iraq's biological campaign. Anthrax would have been one of the agents Hindawi would have been trying to put into production for Baghdad.
Ewen Buchanan, the UNSCOM spokesman, confirmed that Hindawi has been interviewed repeatedly by UNSCOM investigators, though not recently.
Underplaying the importance of Hindawi, Mr Buchanan said he "was not involved in the production phase" and added that UNSCOM has interviewed "hundreds of other people", besides Hindawi.
Other experts, however, suggested that had he successfully defected, Hindawi could have been crucial to UNSCOM's efforts to reveal the truth about Iraq's biological ambitions.
Confirmation yesterday by Mr Straw that a number of "all ports" alerts had been made since the tension in the Gulf, fuelled suspicions among Labour MPs opposed to war against Iraq that the latest reports are part of a conspiracy to wreck the peace deal brokered with Saddam by the Secretary General of the UN, Kofi Annan.
"I am extremely suspicious that this is an attempt to destabilise the agreement over the ceasefire when it seems to be working rather well. It seems to be the action of somebody who wants to fight," said Tam Dalyell, the veteran Labour campaigner.
In a statement to the Commons, Mr Straw said the latest intelligence report of the anthrax threat reached the Government on 6 March, two weeks before the "all ports" alert was issued. Home Office sources said the delay was caused by the need to assess the seriousness of the threat.
Mr Straw told MPs: "Our information is that there is no specific threat to the UK."