Mayawati resigned as Uttar Pradesh chief minister in August

Police have been told to register a case against Mayawati, who will be questioned over plans for a controversial shopping complex next to India's most famous monument, the Taj Mahal.

The inquiry is to be carried out by the country's main federal detective agency, the Central Bureau of Investigation (CBI).

Mayawati has always denied any knowledge of the building project, which was abandoned after a public outcry in July.

Her fragile coalition government in Uttar Pradesh broke up last month amid recriminations over the project.

Shortly afterwards the CBI interviewed her in connection with the plans.

Mayawati, a schoolteacher turned politician, is India's best-known leader of Dalit, or low-caste, groups.

The BBC's Sanjeev Srivastava in Delhi says until recently, as head of the country's most politically crucial state, she had been eyeing a bigger role for herself in national politics.

'Incomplete'

Wednesday's moves by the Supreme Court come two months after it ordered the CBI to investigate the use of public funds for the project, and how it was approved even though it contravened environment protection laws.

However, the court said the CBI's subsequent report was incomplete.

Investigators must establish who approved the discredited scheme

The court has also ordered a number of other people to be investigated, including Uttar Pradesh's former environment minister, Naseemuddin Siddiqui.

Any prosecution of Mayawati and Mr Siddiqui would need the approval of the governor of Uttar Pradesh state.

A number of other figures are under the spotlight for their alleged links to the Taj Mahal building project.

These include the former environment secretary in the national government, VC Misra, and the chairman of the state-owned National Project Construction Corporation, SC Bali.

Work on the Taj shopping complex began last November.

The Uttar Pradesh government said it needed to relocate shops that had been removed from the Taj Mahal premises under a Supreme Court order.

It also argued that tourists would be able to visit the Taj Mahal without travelling through the crowded and polluted streets of Agra city.

The Taj Mahal is a 17th century mausoleum, built by the emperor Shah Jahan for his wife, Mumtaz, who died during childbirth.