This is a new twist in the headscarf dispute, which has been outraging Turkey's secular elite since 2 May.

At the swearing-in ceremony of the new parliament, Merve Kavakci, a newly-elected member for the Islamist Virtue Party, refused to take her headscarf off.

Merve Kavakci did not attend parliament today

She has since been portrayed by local politicians and parts of the media as a dangerous agent of Islamic fundamentalism.

Last week, Turkey's chief prosecutor opened a case to ban the Virtue Party, citing Merve Kavakci as evidence that the Islamists aim to overthrow the secular regime.

Now, the Turkish Foreign Ministry says Ms Kavakci took an oath of allegiance to the United States at the beginning of March, becoming a dual citizen little more than a month before she was elected to parliament.

The prime minister says that should disqualify her from office whether she wears her headscarf or not.

The Turkish constitution does not ban dual citizenship in general, but public servants and holders of public office have to renounce any non-Turkish citizenship before taking up office.

It is unclear whether Ms Kavakci obtained permission from the Turkish authorities to seek US citizenship, and whether she informed them of her dual citizenship while standing for public office.

Virtue Party ban

With her refusal to take off her headscarf, Ms Kavakci, a newly-elected Virtue Party deputy, re-ignited the debate over the place Islam holds in Turkey .

She has refused to back down, with the full support of her party leader, Recai Kutan.

Turkey, which is officially secular, regards the wearing of the Muslim headscarf as a political, pro-Islamic statement and has banned it in many public institutions.

Amid all the commotion, Ms Kavakci has still not taken her oath of office. On Wednesday, she did not attend the first session of parliament since the controversial swearing-in ceremony.

International implications

At the beginning of the week, an international row erupted over Iran's alleged support for the deputy.

The Turkish Foreign Ministry summoned Iran's ambassador to Ankara to complain about a demonstration staged last weekend in which students in Teheran shouted slogans in support of Islamist deputy Merve Kavakci.

Iran quickly responded, saying Turkey should not blame others for its own internal problems.

This is the latest round in a long-running series of disputes between the two neighbouring countries, whose governments are ideologically at opposite ends of the spectrum.