Mr Yeddyurappa was sworn as chief minister on Thursday after the Supreme Court declined to stop him from taking charge but made it clear that his survival in the state's top post would depend on their final verdict.

The Congress-Janata Dal Secular (JDS) combine, which has 116 legislators, had called Karnataka governor's invite "an encounter of the constitution". They petitioned the court that the BJP couldn't claim to have majority unless the BJP triggers defections.

The BJP has 105 lawmakers, including an Independent. It is the largest party but is clearly short of majority. The Congress, which got 78 seats, has partnered with the Janata Dal Secular and together they have 116 seats, four more than the halfway mark.

At the heart of the legal challenge facing Mr Yeddyurappa is a difference of opinion on the parameters that should guide the governor when he, or she, has to invite a political party or formation that stakes claim.

Mr Yeddyurappa's BJP insists it should have the first shot at power by virtue of being the single largest party in the just-concluded Karnataka elections. The Congress insists this isn't good enough.

Because the Congress-JDS alliance has the majority votes, Congress leader Abhishek Singhvi told the court, the only way that the Yeddyurappa government could also cross the half-way mark was by horse-trading. Mr Singhvi, who was representing the alliance in court, called the governor's decision to give Mr Yeddyurappa 15 days to prove his majority when he asked for only 7, "the biggest license to poaching (legislators)".

The three judges, Justices AK Sikri, SA Bobde and Ashok Bhushan, also brushed aside the argument by the central government's top law officer KK Venugopal that the anti-defection law kicks in only after a legislator takes oath of office. This is preposterous, the bench shot back. The anti-defection law bans lawmakers from switching political parties.

The court had put off the hearing by a day because the Congress could not produce the letters submitted by Mr Yeddyurappa to Karnataka governor which had convinced him to invite the BJP, and not the Congress-JDS combine.

To a suggestion that former Attorney General Mukul Rohatgi be asked to get it, Mr Rohatgi had declined, asserting that he was not representing Mr Yeddyurappa but two BJP legislators. The court later ordered the centre and Mr Yeddyurappa to come with the letter at Friday's hearing.