Iran has resumed work on constructing highly sophisticated equipment that nuclear experts say is primarily used for building atomic weapons, according to the latest intelligence reports received by Western diplomats.

The work is aimed at developing the blueprint provided by Dr. A.Q. Khan, the “father” of Pakistan’s nuclear bomb, who sold Iran details of how to build atom bombs in the early 1990s.

Iran’s Revolutionary Guard, which has overall responsibility of the country’s nuclear program, has set up a number of civilian companies to work on the program whose activities are being deliberately concealed from the U.N. nuclear inspection teams.

The companies, based on the outskirts of Tehran, are working on constructing components for the advanced P2 gas centrifuge, which can enrich uranium to weapons grade two to three times faster than conventional P1 centrifuges.

Iran’s controversial nuclear enrichment program at Natanz, which Tehran insists is designed to produce fuel for nuclear power, runs on P1 centrifuges. But Iranian nuclear scientists recently conducted successful tests on a prototype P2 centrifuge at Natanz, and the Revolutionary Guard has now set up a network of companies to build components for the advanced centrifuges.

This has raised concerns among Western experts that Iran is continuing work on its nuclear weapons program, despite Tehran’s protestations that its intentions are peaceful.

“If Iran’s nuclear intentions were peaceful there would be no need for it to undertake this work in secret,” an official familiar with the intelligence reports said.

A previous clandestine attempt by Iran to develop P2 centrifuges was halted in 2004 after the existence of a civilian company set up by the Revolutionary Guard was exposed. U.N. nuclear inspectors found traces of weapons-grade uranium at the company when they inspected the premises.

Reports that Iran has resumed work on sophisticated uranium enrichment technology follow Tehran’s announcement at the weekend that it has no intention of halting its uranium enrichment program at Natanz.

Iranian officials were speaking the day after they had formally submitted their response to a package put together by the world’s leading powers — including Britain — offering a number of incentives in return for halting enrichment.

While European officials yesterday refused to disclose details of the Iranian response, one said that “it was not something that made us jump up and down for joy.”

An Iranian government spokesman said: “Iran’s stand regarding its peaceful nuclear programme has not changed.”

According to recent intelligence reports, President Ahmadinejad of Iran personally ordered the Revolutionary Guard to set up companies for the secret manufacture of components for P2 centrifuges this year.

One of the companies is in a residential building in Amir Abad, western Tehran, where its work is unlikely to be detected by U.N. nuclear inspectors. One of the facilities is said to be run by a company owned by the Revolutionary Guard.

The operation is a direct copy of the Revolutionary Guard’s previous attempt to develop P2 centrifuges, when research work was undertaken by the Kalaye Electric Company, which claimed it was manufacturing watches.

When its true activity was disclosed to U.N. nuclear inspectors in 2004, they found the company had succeeded in building the centrifuges and enriching small quantities of uranium to weapons grade.

Senior officials from Iran’s Atomic Energy Agency are supervising the current clandestine program, which is based on the atomic weapons blueprint sold to Iran by Dr. Khan in 1994.

Reports that Iran is actively working on Dr. Khan’s blueprint will deepen suspicions that Tehran has resumed work on its nuclear weapons program.