Armed with letters of support from several parties, Congress president Sonia Gandhi will meet President A P J Abdul Kalam on Monday, according to reports.
A Congress-led government will be sworn in on Wednesday, the reports said.
The Congress president met her allies on Sunday to work out the modalities of a common minimum programme.
Among those she met was Dravida Munnetra Kazhagam chief M Karunanidhi, who said he would support a coalition government headed by her.
Following the meeting with Gandhi at her 10, Janpath residence in New Delhi, Karunanidhi said in Tamil that he had handed over to the Congress president a letter of support of 16 party MPs addressed to Kalam.
Asked if he was going to join the government, he said, "I did not say we will not join. This is a decision to be taken by the party's working committee."
The DMK leader said the party might take a "few weeks to a few months" to decide on the issue.
The DMK had earlier said that it would not join the government, but had authorised Karunanidhi to take an appropriate decision in this regard.
The Communist Party of India-Marxist also debated the issue of whether to join the government, but could not reach a consensus.
Emerging from a meeting in New Delhi, party leader Jyoti Basu said only four members had spoken and 23 were yet to express their views. "Discussions are still on."
The central committee would take a decision in this regard on Monday, CPI-M sources said.
With inputs from the Press Trust of India