On Sunday, he claimed the ACMD's member from the Royal Pharmaceutical Society had also resigned.
Now the ACMD is due to release a report which could recommend it be banned.
He said "immediate action" would be taken after advice was taken from the ACMD on 29 March.
The Advisory Council on the Misuse of Drugs (ACMD) said there was "insufficient evidence" that khat caused health problems.
When he announced his plans for the group, Prof Nutt said it would take over the science role of the ACMD.
The government also said the ACMD routinely considers evidence from a wide range of sources, including external experts and would continue to do so.
Dr Les King quit the Advisory Council on the Misuse of Drugs (ACMD), saying Home Secretary Alan Johnson had denied Prof Nutt's "freedom of expression".
The herb also triggers withdrawal symptoms such as tiredness, depression, shaking and bad dreams, although the ACMD said this did not necessarily provide evidence of dependency.
Political pressure for a ban soon became irresistible, even though the ACMD had virtually no evidence at all about the pharmacological properties of the drug itself.
The Advisory Council on the Misuse of Drugs (ACMD) said it was concerned about an increase in the number of deaths related to misusing the psychoactive drug.
The Home Office confirmed the ACMD had to have 20 members to remain quorate, and that six positions had to be filled for the advisory group to function.
He also said he had received a "very supportive e-mail" from pharmacology specialist Professor Les Iversen, who was named on Wednesday by the Home Office as the new ACMD chairman.
"The ACMD's review of harms associated with the non-medicinal use of tramadol was prompted by an increasing number of reports within the NHS of tramadol's misuse and harms, " the letter said.
But ACMD chairman Professor Les Iversen said the review "found insufficient evidence of either health or societal harms caused by the use of khat to justify its control in the UK".
The ACMD's chairman Prof Les Iversen called for tramadol to be made a Class C drug, with penalties of up to two years in prison for possession and 14 years for supply.
In 2005 the Advisory Council for the Misuse of Drugs (ACMD) said "the evidence of harm resulting from khat use is not sufficient to recommend its control, " and there was "no evidence of its spread to the general population".
In April it became the first drug to be banned temporarily under new government powers, after which the substance was referred to the Advisory Council on the Misuse of Drugs (ACMD) to consider whether the ban should be made permanent.
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The ACMD said there was "no evidence" khat, which consists of the leaves and shoots of a shrub cultivated in the Horn of Africa and the Arabian Peninsula and contains the stimulant cathinone, was directly linked with serious or organised crime.
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