Around 70 per cent of the income of the six national parties and 51 regional parties came from unknown sources, according to a study that covered the decade to March 2015.

The Association for Democratic Reforms (ADR) study puts such income at Rs 7,832.98 crore. The Congress, which was in power through much of the decade, tops the list with 83 per cent of its total income amounting to Rs 3,323.39 crore coming from unknown sources. The BJP is second with 65 per cent or Rs 2,125.91 crore.

Among regional parties, the Samajwadi Party leads the pack with 94 per cent, followed by the Akali Dal.#

Parties are not required under the law to reveal the source of contributions less than Rs 20,000.