The death of the wife of a prominent former Indian minister and UN diplomat is now being treated as a homicide, Indian police said in a statement.

Sunanda Pushkar, 52, was found dead at the Leela Palace hotel in New Delhi in January 2014. It was initially thought that she had committed suicide.

Pushkar was found dead after rumours spread that her husband Shashi Tharoor was having an affair.

Delhi police chief Bhimsen Bassi said that after an investigation, a medical board had concluded that the woman had been poisoned, AP reported.

It is not yet clear what type of poison was used.

A special team have been set up to further investigate.

"I am anxious to see this case is investigated thoroughly and continue to assure the police of my full cooperation," Tharoor said.

A preliminary report conducted immediately after Pushkar's death failed to reach any conclusion on the cause, but said it was an "unnatural sudden death" and that there were injuries on her body.

The couple married in 2010.