Citing their tax returns, the report by the Association for Democratic Reforms (ADR) said the two parties together got Rs 646.82 crore, or over 77 per cent, of their total income from "unknown sources" during the year.
Voluntary contributions and coupon sales were the key income sources for the ruling BJP and the Congress (INC) while the two parties' cumulative total income amounted to Rs 832.42 crore in fiscal 2016, according to ADR, a non-government organisation that works for electoral reforms.
The findings are based on an analysis of the two parties' income and expenditure data submitted to the Election Commission of India.
In 2015-16, the BJP's income from unknown sources was Rs 460.78 crore while that of the INC was Rs 186.04 crore.
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Income from "unknown sources" refers to those where the source is not declared for donations below Rs 20,000. Such income includes those from sale of coupons, relief fund, miscellaneous income, voluntary contributions and contribution from meetings or morchas, stated the report.
The contribution statements, submitted by the political parties declaring names and other details of donors who contribute above Rs 20,000, are the only known sources of income parties, ADR said.
Among the unknown sources of funding, maximum funds were collected under "voluntary contributions" by the BJP, which amounted to Rs 459.56 crore in fiscal 2016, it said.
The total amount of voluntary contributions, including donations of over Rs 20,000, received by the BJP was Rs 536.41 crore in 2015-16.
As for the Congress, the report said the party collected Rs 167.96 crore from "sale of coupons" during the same period.
Among the seven national parties, the report said the BJP had the highest total income at Rs 570.86 crore in 2015-16, followed by the Congress at Rs 261.56 crore, the CPM (Rs 107.48 crore), the BSP (Rs 47.39 crore), the Trinamool Congress (Rs 34.58 crore), the NCP (Rs 9.14 crore) and the CPI (Rs 2.18 crore).
According to the analysis for 2015-16, 23 per cent of BJP's total income remained unspent whereas it was 26 per cent in the case of the Congress.
The report further said the contribution statements submitted by the political parties declaring names and other details of donors who contribute above Rs 20,000 are the only known sources of income parties.
Funds from "known sources (donations above Rs 20,000)" accounted for Rs 97.27 crore of the two parties' total income in 2015-16 put together.
Of the BJP's total income, Rs 76.85 crore came from such sources whereas the same stood at Rs 20.42 crore in the case of the Congress.
Income from bank interest accounted for Rs 19.61 crore and Rs 44.67 crore for the BJP and the Congress, respectively.
"The BJP declared a total income from donations above Rs 20,000 of Rs 76.85 crore, a mere 14.33 per cent of total donations, from donors whose details are available," the report said.
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