The UK government has flown antidote medicine to the Middle East after some Iraqis became seriously ill from eating cakes laced with the poison thallium.
Two of the victims, both children, died after eating cake delivered to a military club in Baghdad.
Others are being treated in hospital in the Jordanian capital, Amman.
It is the first time the deadly toxin has been used since the downfall of Saddam Hussein, whose regime used it to kill its opponents.
At least two of the poison victims, the secretary of the Iraqi air force club and his daughter, are critically ill in Amman.
They and half-a-dozen other patients suffering from thallium poisoning were flown from Baghdad to Amman as the necessary treatments and antidotes were not available in Iraq.
Britain responded to a request for help from the World Health Organization and medication was flown out.
Thallium is a lethal poison much used by Saddam Hussein’s regime against its opponents. It has not surfaced since his overthrow.
It is an ideal assassin’s tool, being tasteless and easy to administer, and its effects take some time to appear.
It then causes a lingering and painful death. An antidote known as Prussian Blue can be effective if taken quickly.
An investigation is under way in Baghdad, but the affair remains shrouded in mystery. The manager of the air force club told the BBC he believed it was carried out by conspirators with a grudge against the club’s administration.
In what appeared to be a goodwill gesture, a former official delivered two cakes laced with thallium.
They were taken home by two officials and eaten by their families, who all fell ill.