Pakistan is expected to allow United Nations nuclear investigators to put questions in writing to Abdel Qadeer Khan, the scientist at the centre of an illegal smuggling network that supplied nuclear materials and expertise to at least three countries, according to western diplomats.

Such indirect access would fall short of the face-to-face interviews the International Atomic Energy Agency has been seeking but it could still prove an important step in the agency's efforts to untangle the network of manufacturers and middleman that supplied sensitive machinery and know-how to Libya, Iran, North Korea and perhaps others.