National parties collected Rs 11,234 crore in donation from unknown sources from 2004-05 to 2018-19, according to election watchdog Association of Democratic Reforms (ADR).
For this analysis, the ADR said it considered the submission made before the Election Commission of India by seven national parties - BJP, Congress, Trinamool Congress, Communist Party of India (Marxist), Nationalist Congress Party, Bahujan Samaj Party and Communist Party of India (CPI).
The income from unknown sources refers to the funds declared in the Income Tax Returns without naming the source for donations below Rs 20,000.
Such unknown sources include donations via electoral bonds, sale of coupons, relief fund, miscellaneous income, voluntary contributions and contribution from meetings/morchas.
The BSP, however, declared that it did not receive any funds from voluntary contributions (above or below Rs 20,000) from the sale of coupons/electoral bonds or unknown sources of income.
"Between FY 2004-05 and 2018-19, the national parties have collected Rs 11,234.12 crore from unknown sources," the ADR said.
The details of donors of such voluntary contributions are not available in the public domain.
During 2018-19, the BJP declared Rs 1,612.04 crore as income from unknown sources which is 64 per cent of the total income of national parties from unknown sources (Rs 2,512.98 crore), the ADR said.
This is 1.5 times more than the aggregate income from unknown sources declared by the other five national parties (Rs 900.94 crore), it added.
The Congress declared Rs 728.88 crore as income from unknown sources which is 29 per cent of the total income of national parties from unknown sources, the watchdog said.
"Combined income of Congress and NCP from sale of coupons between FY 2004-05 and 2018-19 stands at Rs 3,902.63 crore," it added.