The Indian-born 53-year-old, a former scientist, was single and had no underlying health problems.
Colleagues said that the death of the MP for Middlesbrough South and East Cleveland had come as a “huge shock”.
Sources said that there was no sign of a break in at his home and there appeared to be no suspicious circumstances or third party involved.
Parliamentary staff are believed to have raised the alarm after they were unable to reach Dr Kumar by telephone.
Police officers forced entry to the home in Marton, Middlesbrough, at 12.30pm and found the MP’s body inside.
Dr Kumar, an aide to Hilary Benn, the Environment Secretary, had represented the north east constituency since 1997 and was regarded as a champion of British Steel. He was the fifth MP of Asian origin to be elected in post-war Britain.
The seat, which has a notional Labour majority of 8,000 votes, is expected to remain empty until the forthcoming general election.
Mr Benn led tributes to the “pioneering” politician.
“It is very hard to believe that Ashok is no longer with us,” he said. “Ashok was a pioneer, a doughty fighter for his constituents and a Labour man through and through who cared deeply for others.
“He was also fearless in pursuit of what he saw as right. I came to value his friendship, his loyalty and his sense of fun over the many years we worked together.
“It is why he will be greatly missed by so many of us.”
Announcing the death to the Commons, John Bercow, the speaker, said: “Ashok was a most assiduous Member, much respected by the House and by professional background a very fine chemical engineer.
“I am sure members on all sides of the House will join me in mourning the loss of a colleague and extending our sympathy to the honourable member’s family and friends.”