Germany's Germanischer Lloyd (GL) has stopped verifying safety and environmental standards for Iran's biggest shipping companies, a letter from the company showed, the last big European classification society to pull back as sanctions heat up.

Without verification from such bodies, ships are unable to call at international ports.

It is likely that Iran is already securing cover from Asia-based classification societies, shipping industry sources said.

NITC lost its ship insurance cover from European providers last year due to the sanctions. A senior NITC official said last week it had $1 billion in alternative ship insurance cover, especially from private Iranian firm Kish P&I, to keep its fleet on the water.

The NITC official added that it also had alternative ship classification cover, without giving further details.