Sources said that Tharoor, who is in Kerala, has been asked to join probe at the earliest.
"A legal notice has been issued to Tharoor to join the murder probe. The questioning will be held under section 160 CrPC," said a senior police official.
Earlier on Wednesday, Tharoor had accused Delhi Police of "assaulting" and "intimidating" his domestic help into "confessing" that they both killed Sunanda Pushkar as police said it believes prima facie it is a case of murder.
"I was therefore shocked and appalled to learn that in the course of the 16-hour interrogation conducted by four Delhi Police officers on Friday (7/11/14) and again during the 14- hour interrogation on Saturday (8/11/14), my domestic helper Narayan Singh was repeatedly physically assaulted by one of your officers.
"Worse, the officer used the traumatic physical assault to try and intimidate Narayan into 'confessing' that he and I murdered my wife," Tharoor said in the letter.
Tharoor also referred to his telephonic conversation with Delhi Police Commissioner B S Bassi on November 8 when he had expressed his concerns about the alleged police action.
51-year-old Sunanda was found dead in a five-star hotel in the national capital on January 17 last year.
The AIIMS report had concluded that her death was unnatural and due to poisoning basede on which police registered a case of murder in Sunanda's death.
Dr Sudhir Gupta, the head of the AIIMS medical board, which had done the postmortem, maintained that their report did not mention "homicide" in the findings that held "poisoning" as the cause of death.
"Our report doesn't mention the death is homicidal in nature. It is due to poisoning. Police had sought an opinion in the matter and we have already given it. Now it is for the police to probe further," head of the medical board Sudhir Gupta said.
Pushkar was admitted to Kerala Institute of Medical Sciences in Thiruvananthapuram last year between 12th-14th January 2014. She was found dead in the five-star hotel in Delhi three days later.
Her family had said that she was suffering from lupus and was taking heavy medication. However, AIIMS doctors said that she was not suffering from lupus and that she was a healthy individual.
Later, a spokesperson of the hotel denied media reports that CCTV footage of the hotel was found missing.
"There have been some incorrect reports suggesting missing Hotel CCTV footage regarding Sunanda Tharoor's demise at The
Leela Palace New Delhi," the spokesperson told PTI.
"We wish to clarify that all 230 surveillance cameras on the hotel premises, including all 10 cameras on the floor of the incident, were fully operational and the entire CCTV footage covering the complete period of Tharoor's stay, from Jan 15–17, 2014, was handed over to the police and the receipt was duly acknowledged," he said.
The spokesperson also said that the hotel has complied with every request made by authorities and continues to extend its cooperation to the fullest.
He also clarified that Sunanda and her husband, then Union Minister Shashi Tharoor were shifted to the suite from their earlier rooms as they had requested for more spacious accommodations.
"The comfort of our guests is always foremost in our mind and we follow only the best practices to ensure guest safety and security. The guest requested more spacious accommodations and they were moved to a bigger room accordingly," he said.