Sister Nirmala, the nun chosen by Mother Teresa to carry forward her legacy of service to the needy, passed away on Tuesday at the age of 80, six years after stepping down as the Superior General of the Missionaries of Charity.

Sister Nirmala passed away at 12.05am amid prayers and hymns at the Missionaries of Charity's Contemplative House, near St. John's Church in Sealdah. She had refused to be hospitalised despite not keeping well.

Sources said Sister Nirmala, born Kusum Joshi at Doranda in Ranchi on July 23, 1934, died of heart failure associated with kidney malfunction. She had been suffering from age-related ailments for some time.

"When Mother Teresa died, she left behind a legacy of self-sacrifice and devotion to the poor. Many thought that it would be difficult for the order to sustain the legacy. But Sister Nirmala took the legacy forward with quiet dedication. She was a nun whose one and sole desire was to serve the poor," said Most Revered Archbishop Thomas D'Souza, paying tribute to her at St John's Church.

Sunita Kumar, one of Mother Teresa's closest associates, called Sister Nirmala "exceptionally affectionate".

"I have known Sister Nirmala for many many years, even when Mother (Teresa) was alive.... She was doing Mother's work the way Mother wanted," she said.

Visitors will be allowed a glimpse of Sister Nirmala's body at Mother House between 10am and 3pm on Wednesday. The funeral mass is scheduled for 4pm, followed by interment at St John's Church.

Sister Nirmala had led the Missionaries of Charity for two terms before declining to continue as Superior General in 2009 because of age and health.

The eldest of 10 children in a Nepali Hindu family, she had first met Mother Teresa in March 1958 and joined the Missionaries of Charity the same year.

Sister Nirmala, who had a master's degree in political science from Patna Women's College, was elected superior general of the Missionaries of Charity in March 1997, six months before Mother's death. Although an election to the post was held, Mother Teresa's faith in Sister Nirmala had sealed the choice long before the result.

A quote attributed to Mother Teresa and put up on a board at Mother House along with photographs of Sister Nirmala encapsulated what she thought of her successor. "If God could find someone little like me, that means He can find someone even smaller," Mother had been quoted as saying on the day of Sister Nirmala's election.

Sister Nirmala made way for Sister Prema in 2009, returning to a life of prayer after 12 years as superior general. She was conferred the Padma Vibhushan the same year.

"She (Sister Nirmala) gave us the spirit of Mother Teresa and she continued her work in a very beautiful, peaceful and uniting way... She had been very important to our sisters - in guiding us in our service to the poorest of the poor and also in our prayers to deepen our relationship with God," Sister Prema said.

Sister Nirmala had been hospitalised around three months ago but spent her last days in the Contemplative House even as her health deteriorated. "She refused to be admitted to a hospital (again)," recalled Father Moloy D'Costa, the parish priest of St John's Church.

Chief minister Mamata Banerjee and Prime Minister Narendra Modi tweeted their admiration for the nun. "Sister Nirmala's life was devoted to service, caring for the poor & underprivileged. Saddened by her demise. May her soul rest in peace," the Prime Minister wrote.

Mamata, who visited St John's Church in the afternoon to pay tribute, had tweeted in the morning: "Saddened at the passing of Sister Nirmala who headed Missionaries of Charity after Mother Teresa. Kolkata and the world will miss her."

Surendra Prasad Joshi, Sister Nirmala's younger brother, had arrived in Calcutta from Kathmandu hours before her death.

"She was dedicated to helping the needy and the poor. She was influenced by Mother Teresa and came to Calcutta to join the Missionaries of Charity. Our father didn't approve of it but he came to Calcutta and saw the activities of the Missionaries of Charity and he accepted it," Joshi said.