India's top court says private companies will have to return most of the coal blocks allocated to them by the government since 1993.

Wednesday's order comes a month after the court said that government allocations of coal reserves to private companies from 1993 to 2010 were illegal.

The court said the companies must relinquish the coal fields by March next year.

About 218 coal blocks, or areas of unmined reserves, were allocated to companies for use in power plants and factories.

The court's ruling extends beyond the initial case, dubbed "Coalgate" by the Indian media, in which the previous Congress party-led government was accused of costing the treasury hundreds of billions of dollars by selling or allocating about 155 coal blocks in 2004-09 without competitive bidding.