A veteran leader of India's main opposition Congress party quit on Friday, issuing a scathing resignation letter in which he blamed the scion of the influential Gandhi family for the decline of the party that dominated Indian politics for decades.

After ruling for much of India's post-independence period, the 137-year-old party, which traditionally promoted a secular state, has floundered against Prime Minister Narendra Modi and his Hindu nationalist Bharatiya Janata Party (BJP).

The BJP has won two successive general elections since 2014 with landslides and looks well-placed to win the next one, due by 2024.

Ghulam Nabi Azad, a former federal minister who also ruled the disputed region of Kashmir, said he was leaving Congress after nearly 50 years and blamed the party's de facto second-in-command, Rahul Gandhi, the son of party chief Sonia Gandhi, for its failings.

"Unfortunately after the entry of Shri Rahul Gandhi into politics and particularly after January, 2013 when he was appointed as vice president by you, the entire consultative mechanism which existed earlier was demolished by him," Azad said in a letter to Sonia, using an honorific for her son.

"All senior and experienced leaders were sidelined and new coterie of inexperienced sycophants started running the affairs of the party."

Rahul Gandhi was not available for comment.

Congress spokesman Jairam Ramesh questioned the timing of Azad's letter, saying it came as the party was working to organise nationwide campaigns against price rises, employment and religious polarisation.

"It is most unfortunate, most regrettable that at this point of time we've had to read this letter that has been released to the press," Ramesh said.

The son and grandson of former prime ministers, Rahul Gandhi took over as Congress president in 2017 and fought unsuccessfully to stop Modi from winning a second term.

He quit the leadership of the party in July 2019 after months of vacillation but remains a hugely influential figure within the Congress, a fact that Azad said had stalled any significant internal reforms.

"Unfortunately, at the national level we have conceded the political space available to us to the BJP and state level space to regional parties," Azad said.

"This all happened because the leadership in the past 8 years has tried to foist a non-serious individual at the helm of the party."

The BJP is expected to win a third straight term in the next election unless disparate opposition parties are able to come together to overcome Modi's popularity.