The fast-growing Islamic bond industry has been seized by a fit of religious doubt.

The Islamic credentials of the bonds, called sukuks, have faced growing questioning in recent months, forcing financial engineers back to the drawing board in search of structures more compliant with Islam.

Criticism of sukuks as "un-Islamic" was first voiced last year by Sheikh Muhammad Taqi Usmani, a prominent religious scholar who heads the Bahrain-based Accounting and Auditing Organisation for Islamic Financial Institutions.

Bankers say the views reflected a growing unease among religious scholars, whose support for sukuks has fuelled phenomenal growth in the industry.

"It [the criticism] has had a huge impact," said Hussein Hassan, head of Islamic finance at Deutsche Bank, on the side lines of a Euro-money conference in London. "A lot of structuring of sukuks was put on hold until the issue was clarified."

After consultations with bankers, the 18-member Sharia, or Islamic law, board of the AAOIFI will have its say on the sukuk debate next week, according to Mohamad Nedal al-Chaar, secretary-general of the organisation.

Total sukuk issuance surged 73 per cent last year to more than $47bn, according to the Islamic Finance Information Service, a data provider. Sukuk issuance has reached $1.3bn this year, according to Islamic Finance Information Service. But some bankers say sukuks will now require more stringent structures to appeal to buyers, regardless of the AAOIFI ruling.

The debate over the purity of sukuks underlines the wider problem of a lack of standardisation in Islamic finance. Each financial institution relies on its own Sharia board to approve products. Different scholars can disagree on what is "Islamic", even within one country.

Sukuks come in different structures but the most popular form in the past two years involves a repurchase undertaking where the iss-uer promises to pay back the face value of the bond when it matures or in the event of a default.