The sudden resignation of India's Vice-President Jagdeep Dhankhar on Monday night has sparked speculation about what prompted the move.

Dhankar, also the chairman of the upper house of the parliament, announced his resignation on the first day of the monsoon session, citing health concerns.

The ruling Bharatiya Janata Party (BJP), which had nominated him for the post, has yet to comment on his resignation.

But many opposition leaders have expressed surprise at the move, with one MP saying there was more to it "than meets the eye".

Dhankhar, 74, took office in August 2022, with his term set to run until 2027.

On Monday, Dhankhar presided over a Rajya Sabha - the upper house - session and swore in new members.

He also received an impeachment notice against high court judge Yashwant Varma, signed by over 50 MPs, following the discovery of cash at the judge's home earlier this year. Justice Varma has denied any wrongdoing by him or his family.

Dhankhar's official schedule had listed a visit to the western city of Jaipur on Wednesday in his capacity as vice president.

But on Monday evening, he submitted his resignation letter to President Droupadi Murmu, saying he wanted "to prioritise health care and abide by medical advice".

"It has been a privilege and satisfaction to witness and partake in India's remarkable economic progress and unprecedented exponential development during this significant period. Serving in this transformative era of our nation's history has been a true honour," Dhankar said in the letter.

The abrupt move sent ripples through political circles, with some remarking that they had spoken to Dhankhar just hours before his resignation.

"What is going on in this government?" asked Anand Dubey, spokesperson of the Shiv Sena (UBT) party.

"This decision came without proper consultation or discussion. If health was the concern, the resignation could have been submitted a few days before the session or even after," he said.

Manish Tewari, an MP of India's main opposition Congress party, called the move "totally unexpected".

"No doubt Mr Dhankhar has to give topmost priority to his health," he said.

"But, clearly there is far more to his totally unexpected resignation than meets the eye."

Jairam Ramesh, another Congress MP, called Dhankar's resignation "unprecedented".

He also pointed to the absence of two ministers from an advisory committee meeting presided over by Dhankar that was supposed to happen on Monday afternoon.

Ramesh said Dhankar was not personally informed of their absence and took "umbrage" before rescheduling the meeting for Tuesday.

"Jagdeep Dhankar's resignation speaks highly of him. It also speaks poorly of who got him elected as vice president in the first instance," he added.

Dhankar, a lawyer by profession, first entered politics as an MP from Rajasthan's Jhunjhunu seat in 1989.

In 2003, he joined the BJP and went on to head the party's legal department. In 2019, he was appointed the governor of West Bengal state where he was often at loggerheads with the state government, run by the Trinamool Congress party.

He resigned as governor in 2022 after the BJP nominated him as vice president.

Dhankhar underwent angioplasty in Delhi in March but quickly resumed his duties and remained active in parliament.

With the vice president's seat now vacant, his responsibilities are likely to be handled by the deputy chairperson of the Rajya Sabha or another member designated by the president.