Insurance cover for the Ground Zero clean-up project could become the largest insurance contract in history.

The wrap-up policy clinched by Liberty Mutual covers four major contractors, dozens of subcontractors and more than 1,000 workers at the site under one insurers' plan for general liability and workers' compensation.

The project will involve removal of 1.2 million metric tons of debris from four collapsed buildings, three partially collapsed buildings and nine buildings with major structural damage.

An AM Best analyst said: "Liberty has not revealed the extent of the premium, but it is likely to be one of the largest contracts ever."

Insurance Information Institute chief economist Robert Hartwig said the contract was bigger than that of the "Big Dig" in Boston, a massive transport infrastructure project.

According to industry sources, total premiums for the Boston project will exceed $670m (£475m).