The TDS wing of the Income Tax Department has unearthed defaults of over Rs 3,500 crore by a major oil company, and a telecom operator in Delhi, the Central Board of Direct Taxes (CBDT) said in a statement on March 4.

Calling these a "major breakthrough", the CBDT said that a TDS default of Rs 324 crore by a telecom operator in Delhi and another worth around Rs 3,200 crore by a major oil company have been detected.

Sources told CNBC-TV18 that the telecom operator and the oil major under the tax scanner are Bharat Sanchar Nigam Limited (BSNL) and Indian Oil.

The TDS survey conducted by the tax department unearthed huge defaults in deduction and deposit. It found that several hospitals in Delhi had been openly flouting TDS and TCS norms.

A premier hospital in the city was found to have been deducting tax at the rate of 10 percent on salary paid to the doctors, instead of the present TDS rate of 30 percent applicable on salary payments.

The survey uncovered that another hospital in Delhi was not paying any TDS on construction contracts as per statutory requirements. The release noted that TDS defaults of Rs 70 crore and Rs 20 crore, respectively were detected in the said hospitals.

Several hospitals, as per the survey, are still not collecting the required 1 percent tax on cash payments of over Rs 2 lakh. Meanwhile, it was also found that a Delhi-based real estate developer has not been depositing TDS over many years.

The department detected similar irregularity or TDS default in the case of a real estate group of Delhi against which it conducted a survey early this week.

"Verification and analysis indicated outstanding TDS liability and interest payable of Rs 214 crore," the CBDT said in the statement.