The Congress has suffered a political setback following the resignation of Mr Jyotiraditya Scindia and 22 legislators in Madhya Pradesh state, deepening an existential crisis for a party that is struggling for political relevance in modern Indian politics.

Mr Scindia, 49, an articulate leader, yesterday joined Prime Minister Narendra Modi's Bharatiya Janata Party (BJP), with legislators loyal to him expected to follow suit. The move could lead to the collapse of the Congress-led Madhya Pradesh government.

That would give the BJP a chance to form the government in the Hindi heartland state, which is seen as key objective for the party. The BJP has always drawn its strength from the Hindi heartland and used it to extend its reach in the country.

Once seen as close to Congress leader Rahul Gandhi, Mr Scindia is said to have been unhappy for some time over not being chosen as Madhya Pradesh chief minister. The post went to Mr Kamal Nath, a senior Congress leader.

Mr Scindia belongs to the erstwhile royal family of Gwalior whose members have served on both sides of the political divide. His father Madhavrao Scindia was a prominent Congress leader but his aunts, including former Rajasthan chief minister Vasundhara Raje, are members of the BJP.

Mr Scindia, in his resignation letter to Congress president Sonia Gandhi, said: "While my aim and purpose remain the same as they have always been from the beginning, to serve the people of my state and country, I believe I am unable to do this anymore with this party."

The letter was posted on Twitter.

The BJP has nominated him to the Upper House of Parliament.

The resignation of Mr Scindia, considered one of Congress' bright stars, comes amid ferment within what was once the grand old party of Indian politics, which led the fight for independence from British colonial rule.

The party suffered back-to-back losses in the general election in 2014 and last year, and saw its political influence wane across India.

Mr Gandhi quit as Congress president shortly after the election results last year and his mother Sonia, who is known to be in poor health, took over the role. The electoral losses have seen many senior Congress leaders speak out.

Mr Shashi Tharoor, a former Congress foreign minister and United Nations under-secretary-general, said the time has come to change the public perception that the party is adrift, with voters driven towards other political alternatives.

Another Congress leader, Mr Jairam Ramesh, took aim at the old guard leaders, suggesting that they were unwilling to cede power to the next generation.

Still, Congress has put on a brave front after Mr Scindia's resignation, with Mr Kamal Nath expressing confidence that his government would survive the upheaval.

Congress leader Digvijay Singh also maintained that 13 of the 22 legislators were not keen on joining the BJP.

Many analysts, however, see the resignation as a boost for the BJP.

"It will help reinvigorate the BJP, especially if they manage to form the government in Madhya Pradesh after facing a defeat in Delhi," said Dr N. Bhaskara Rao, who is the chairman of the Centre for Media Studies.

He said: "Scindia is not a mass leader. But it will have a demoralising impact on the Congress and its cadres, junior and senior. There is something dramatically wrong within the Congress, and it is just not able to come out of its decline."