Singhal, who was referred to Medanta - The Medicity hospital here in Sector 38, is being kept on life support in the Intensive Care Unit, a doctor at the hospital told on the condition of anonymity.
Singhal was admitted late on Friday night after he complained of breathlessness.
On being contactedby media, doctors from the multi-specialty hospital refused to give an official statement.
Singhal was also referred to this hospital around a fortnight back, but he was discharged after he showed signs of improvement. He was again brought back as his condition deteriorated.
Sources told that Singhal has been suffering from heart and breathing problems.
A statement issued by the VHP said: "According to doctors, his condition is critical and he is under constant observation."
BJP president Amit Shah, union Health Minister J.P. Nadda, BJP general secretary (organisation) Ramlal visited the hospital and inquired about Singhal's health.