NEW DELHI - India's premier probe agency has questioned former prime minister Manmohan Singh in connection with a coal scam, in a huge embarrassment for the country's main opposition Congress party, sources said on Wednesday.

Singh, who was the Indian prime minister for a decade till the Congress party was routed by the Bharatiya Janata Party (BJP) in the general elections last year, was questioned by a team of top officials of the Central Bureau of Investigation at his residence in the national capital last Sunday, according to sources.

The former prime minister was questioned following a direction from the country's Supreme Court.

Singh also held the Coal Ministry when the coal scam, dubbed as coal gate by the Indian media, took place. The government auditor has alleged that state-owned coal fields were undersold to private companies, thus causing a huge loss to the exchequer.