NEW DELHI: Indian federal investigators questioned on Friday a former Indian minister at the heart of one of the biggest corruption cases in the country's history.

Ex-telecom minister A. Raja quit last month over the scandal involving the cut-price sale of second-generation (2G) mobile phone licences that India's government audit watchdog said cost the treasury as much as 40 billion dollars.

A Central Bureau of Investigation (CBI) spokesman said Raja was being questioned at the CBI headquarters in New Delhi.

"We are talking to him about the allocation of 2G spectrum," the spokesman told AFP, adding that it was "very difficult" to say how long the interrogation would last.

Raja is accused of changing rules to favour certain companies, many of which, according to the auditor, were ineligible to apply for wireless airwaves.

Raja has denied the allegations.

The 2G scandal paralysed parliament for the entire winter session and has brought calls for Prime Minister Manmohan Singh's resignation from the main opposition Bharatiya Janata Party.