NEW DELHI  - India's opposition vowed on Wednesday to block parliamentary proceedings until the prime minister resigned over a coal scandal, raising the prospect of more legislative deadlock and stalled reforms.Prime Minister Manmohan Singh was implicated by the national auditor in a report published last Friday which suggested the government had lost out on billions of dollars of revenue by gifting away coal mining rights.Singh, who has seen his reputation as "Mr Clean" damaged by a string of scandals in his second term in government, served as acting coal minister from 2004-2009.Senior opposition leader Arun Jaitley said his Bharatiya Janata Party (BJP) would obstruct parliamentary proceedings until the 79-year-old Singh took personal responsibility and stepped down."Parliamentary obstructionism is normally to be avoided, but in rare cases parties do adopt it," Jaitley told the Times Now television channel in an interview as both houses of parliament were adjourned for the second day in row.“The way forward is that the prime minister must accept his culpability. The reason is very obvious. For five out of the eight years (analysed by the auditor)... he himself was the coal minister,” Jaitley added.The disruption in parliament comes as India's economy is badly faltering, with investors and business leaders looking for decisive action from the under-fire government to restore confidence and spur expansion.Quarterly economic growth slumped to its lowest level in nine years in the first three months of the year and more bad GDP data are expected at the end of this month.In 2010, the BJP and opposition allies blocked an entire session of parliament, forcing adjournments for 22 business days in a row which meant no legislation was passed."BJP goes for broke, wants PM to quit," headlined The Times of India on its front page on Wednesday, underlining how the opposition had staked its credibility on removing Singh.President of the ruling Congress party, Sonia Gandhi, is expected to stand steadfastly by her choice as premier, who has weathered repeated calls from the opposition to resign previously.The report by the Comptroller and Auditor General (CAG) said private companies had made windfall gains of about $33.4 billion since 2004 after being given mining rights via a process that "lacked transparency and objectivity".The government had known of the huge profits for private operators, but had failed to introduce an open bidding process that would have brought in revenue for the national exchequer, the CAG said.The report did not contain any allegations of corruption or criminal practices and the government accused the CAG, an independent constitutional body, of publishing spurious figures."I think as and when this case is investigated more glaring facts will come out," Jaitley claimed.Parliamentary Affairs Minister Pawan Kumar Bansal told reporters the government was ready for a discussion about the CAG report in parliament and accused the BJP of running away from an open debate."Why are they even opposing a discussion? They are scared about the fact that when this discussion takes place, skeletons will come out," he said.The government has said its efforts to introduce open bidding for coal resources were held up by opposition from some state governments, notably ones run by the BJP.The "coalgate" scandal centres on the opaque system of gifting away valuable coal resources to companies, with little known about why politicians and bureaucrats favoured some operators over others.In 2010, Singh's government was hit by a huge scandal over the murky sale of mobile phone spectrum, which led to the arrest of a minister accused of taking kickbacks in return for favours.