NEW DELHI (Reuters) - Indian federal agents questioned a former telecoms minister over the countrys biggest corruption case on Friday, a move the government hopes will help ease a crippling political row with the opposition. Andimuthu Raja is being investigated over his role in the sale of telecoms licences, which a government audit says may have been given too cheaply to ineligible firms in 2007-8, causing a potential government revenue loss of $39 billion. The row has led to the most serious political standoff in recent times in India with an assertive opposition disrupting the last session of parliament which ended this month, demanding a joint inquiry which would have the power to summon the prime minister for questioning. The standoff has put policymaking in limbo and economic reforms have been delayed in Asias third-largest economy. R.K. Gaur, spokesman of the Central Bureau of Investigation (CBI), said Raja was being questioned at the agencys New Delhi office. Although there is little threat to the Congress party-led government, a series of scandals has eroded its image since an impressive election victory last year and has become a test of how Prime Minister Manmohan Singh tackles widespread corruption. Raja, a member of the DMK party from south India which is a key member of the coalition, denies any wrongdoing. Corruption is widespread in India and prosecutions are rare, particularly of high government officials or political leaders. The worlds second-fastest growing major economy ranks 87th on graft watchdog Transparency Internationals list based on perceived corruption a worse rating than rival China. Indias Supreme Court had criticised the CBI for not questioning Raja in its year-long investigation despite him being at the centre of the inquiry. The countrys top court has also dragged the prime minister into the scandal by forcing him to answer questions on why he was slow to act against Raja.