By Charles Haviland
BBC News, Kabul
The former head of broadcast media for the Taleban has arrived in the Afghan capital, Kabul, and is likely to join the Western-backed government there.
Many former Taleban leaders have defected to Mr Karzai's government
There has been no immediate comment from the government.
The Taleban and a former mujahideen commander who brokered the deal have confirmed the defection.
Under Afghanistan's Islamist Taleban government, from 1996 to 2001, Ishaq Nizami was well-known as head of the television and radio directorate.
In practice there was only radio and Mr Nizami, who never had a military role, made regular broadcasts in support of the Islamic movement's leader, Mullah Omar.
After the US-led invasion of the country, which led to the overthrow of the Taleban authorities, he disappeared from public view and stopped having a media role with the Taleban, who started fighting an insurgency.
He has now resurfaced in Kabul, according to the man who brokered the deal, former mujahideen commander Sadeq Mohammand.
[Mr Nizami] worked to protect some of the country's music and film archives, despite the ban on both art forms at the time
Local media quoted Ishaq Nizami as saying he respects Afghanistan's new constitution and will encourage other Taleban to join the government of President Hamid Karzai.
A spokesman for the Taleban has confirmed the defection and accused Mr Nizami of being brainwashed.
Several dozen former Taleban leaders, including their former foreign minister, have returned to Kabul under a national reconciliation process.
It is not yet clear how much influence, if any, Mr Nizami will have in bringing about other similar defections.
Some people who worked in media under the Taleban in Kabul have said that he worked to protect some of the country's music and film archives, despite the ban on both art forms at the time.