The development comes ahead of civic body polls and bypolls for two assembly seats in Rajasthan and has increased the tally of the Congress in the 200-member Assembly to 106.
The House has an effective strength of 198 as two seats are vacant. Hitting out at the Ashok Gehlot government, BSP President Mayawati said the Congress has once again proved that it is "unreliable" and "untrustworthy".
MLAs Rajendra Singh Gudha, Jogendra Singh Awana, Wajib Ali, Lakhan Singh Meena, Sandeep Yadav and Deepchand met Assembly Speaker CP Joshi on Monday night and handed over a letter to him stating their decision to join the Congress.
BSP legislators were earlier supporting the Ashok Gehlot government from outside and were in constant touch with the chief minister. "All of us decided to join the Congress to strengthen the government in the interest of the state.
We first met the chief minister and later gave a letter about our decision to the assembly speaker," one of the MLAs, Joginder Singh Awana, told reporters here on Tuesday.
Mayawati termed the move as a "betrayal". "The Congress government in Rajasthan has once again broken BSP MLAs and proved that it is an unreliable and untrustworthy party," she said in a tweet. "The Congress, instead of fighting against its bitter opponents and organisations always work to harm those parties which cooperate/support them.
The Congress is thus an anti-SC/ST/OBC party and has never been sincere and honest about the right to reservation of these classes," the former Uttar Pradesh chief minister said in another tweet.
In a fresh tweet, the BSP chief said, "The Congress has always been opposed to Dr Bhimrao Ambedkar and his humanitarian ideology. For this reason, Dr Ambedkar had to resign as the first law minister of the country. The Congress neither allowed him to get elected to the Lok Sabha nor conferred him with Bharat Ratna. Very sad and shameful."
The latest political development in Rajasthan is expected to provide more stability to the Gehlot government which came to power in December last year.
The 68-year-old Gehlot had displayed similar political skills in 2009 by bringing all the (then) six BSP MLAs to the Congress fold in order to make his government stable. The then Congress government led by Gehlot was five short of a majority and by managing to bring BSP MLAs in Congress, the chief minister had increased the tally of the Congress to 102 in the House.
Gudha is the only sitting MLA who was among the (then) six BSP MLAs to defect to the Congress in 2009.
He had levelled corruption charges against Mayawati last month alleging that tickets to contest elections are given in return for money in the BSP.
The Congress now has a tally of 106 and its alliance partner Rashtriya Lok Dal (RLD) has one seat.
The ruling party also enjoys outside support of 12 of the 13 Independent MLAs.