According to AIIMS sources, some blockages have been found in his heart, and doctors are contemplating the next course of action, they said.
The 76-year-old Singh had visited AIIMS on Tuesday for an "overall medical checkup," an official statement had said. After undergoing various cardiac tests, the prime minister returned to his residence.
AIIMS Medical Superintendent Dr D K Sharma had said the prime minister had come for a "checkup" which was conducted by a team of doctors from various disciplines.
Singh had cancelled his visit to Kolkata, scheduled for Wednesday, where he was to attend the Asiatic Society's post-bicentennial silver jubilee celebrations and inaugurate the 150th foundation day of St Xavier's College.
The sources said Singh, who had earlier undergone a bypass surgery, had complained of chest pain after which a decision to carry out the tests was taken.
While doctors had stated there was no cause for worry, the hospital, as a precautionary measure, kept a VVIP operation theatre ready.
A special team of doctors from various disciplines has been formed. The doctors have been briefed by the elite Special Protection Group, which guards the prime minister, about the necessary precautions relating to his security.