Ratna Singh, 60, is the daughter of former Union minister Dinesh Singh who is known for his association with the Gandhi family. She was said to be upset with Congress General Secretary Priyanka Gandhi Vadra.
Ratna Singh won the 1996, 1999 and 2009 Lok Sabha elections from Pratapgarh on a Congress ticket.
With her joining the BJP, the Congress is now in deep trouble in an area that was once known as its stronghold.
Rae Bareli, which is Congress interim President Sonia Gandhi's parliamentary constituency, is flanked by Pratapgarh on one side and Amethi on the other.
Amethi has already been lost to the BJP when Union minister Smriti Irani won the Lok Sabha poll, defeating then Congress president Rahul Gandhi.
In Rae Bareli, two Congress legislators, Aditi Singh and Rakesh Singh, have already turned hostile and it is a matter of time before they formally join the BJP. Both the legislators had defied the Congress whip and had participated in the special Assembly session on October 2.
With Ratna Singh joining the BJP in Pratapgarh, the Congress now stands completely isolated in the region.
Earlier, Congress MP Sanjay Singh had also quit the Congress to join the BJP. Sanjay Singh belonged to Amethi and had contested elections from Sultanpur.
( With inputs from IANS )