Former Indian prime minister Manmohan Singh has died at the age of 92.

Singh was one of India's longest-serving prime ministers and he was considered the architect of key liberalising economic reforms, as premier from 2004-2014 and before that as finance minister.

He had been admitted to a hospital in the capital Delhi after his health condition deteriorated, reports say.

Singh was the first Indian leader since Jawaharlal Nehru to be re-elected after serving a full first term, and the first Sikh to hold the country's top post. He made a public apology in parliament for the 1984 riots in which some 3,000 Sikhs were killed.

But his second term in office was marred by a string of corruption allegations that dogged his administration. The scandals, many say, were partially responsible for his Congress party's crushing defeat in the 2014 general election.

Singh was born on 26 September 1932, in a desolate village in the Punjab province of undivided India, which lacked both water and electricity.

After attending Panjab University he took a master's degree at the University of Cambridge and then a DPhil at Oxford.

While studying at Cambridge, the lack of funds bothered Singh, his daughter, Daman Singh, wrote in a book on her parents.

"His tuition and living expenses came to about £600 a year. The Panjab University scholarship gave him about £160. For the rest he had to depend on his father. Manmohan was careful to live very stingily. Subsidised meals in the dining hall were relatively cheap at two shillings sixpence."

Daman Singh remembered her father as "completely helpless about the house and could neither boil an egg, nor switch on the television".

Singh rose to political prominence as India's finance minister in 1991, taking over as the country was plunging into bankruptcy.

His unexpected appointment capped a long and illustrious career as an academic and civil servant - he served as an economic adviser to the government, and became the governor of India's central bank.

In his maiden speech as finance minister he famously quoted Victor Hugo, saying that "no power on Earth can stop an idea whose time has come".

That served as a launchpad for an ambitious and unprecedented economic reform programme: he cut taxes, devalued the rupee, privatised state-run companies and encouraged foreign investment.

The economy revived, industry picked up, inflation was checked and growth rates remained consistently high in the 1990s.

Manmohan Singh was a man acutely aware of his lack of a political base. "It is nice to be a statesman, but in order to be a statesman in a democracy you first have to win elections," he once said.

When he tried to win election to India's lower house in 1999, he was defeated. He sat instead in the upper house, chosen by his own Congress party.

The same happened in 2004, when Singh was first appointed prime minister after Congress president Sonia Gandhi turned down the post - apparently to protect the party from damaging attacks over her Italian origins. Critics however alleged that Sonia Gandhi was the real source of power while he was prime minister, and that he was never truly in charge.

The biggest triumph during his first five-year term was to bring India out of nuclear isolation by signing a landmark deal securing access to American nuclear technology.

But the deal came at a price - the government's Communist allies withdrew support after protesting against it, and Congress had to make up lost numbers by enlisting the support of another party amid charges of vote-buying.

A consensus builder, Singh presided over a coalition of sometimes difficult, assertive and potentially unruly regional coalition allies and supporters.

Although he earned respect for his integrity and intelligence, he also had a reputation for being soft and indecisive. Some critics claimed that the pace of reform slowed and he failed to achieve the same momentum he had while finance minister.

When Singh guided Congress to a second, decisive election victory in 2009, he vowed that the party would "rise to the occasion".

But the gloss soon began to wear off and his second term was in the news mostly for all the wrong reasons: several scandals involving his cabinet ministers which allegedly cost the country billions of dollars, a parliament stalled by the opposition, and a huge policy paralysis that resulted in a serious economic downturn.

LK Advani, a senior leader in the rival BJP party, called Singh India's "weakest prime minister".

Manmohan Singh defended his record, saying his government had worked with "utmost commitment and dedication for the country and the welfare of its people".

Singh adopted the pragmatic foreign policies pursued by his two predecessors.

He continued the peace process with Pakistan - though this process was hampered by attacks blamed on Pakistani militants, culminating in the Mumbai gun and bomb attack of November 2008.

He tried to end the border dispute with China, brokering a deal to reopen the Nathu La pass into Tibet which had been closed for more than 40 years.

Singh increased financial support for Afghanistan and became the first Indian leader to visit the country for nearly 30 years.

He also angered many opposition politicians by appearing to end relations with India's old ally, Iran.

A studious former academic and bureaucrat, he was known for being self-effacing and always kept a low profile. His social media account was noted mostly for dull entries and had a limited number of followers.

A man of few words, his calm demeanour nevertheless won him many admirers.

Responding to questions on a coal scandal involving the illegal allocation of licences worth billions of dollars, he defended his silence on the issue by saying it was "better than thousands of answers".

In 2015 he was summoned to appear in court to answer allegations of criminal conspiracy, breach of trust and corruption related offences. An upset Singh told reporters that he was "open for legal scrutiny" and that the "truth will prevail".

After his time as premier, Singh remained deeply engaged with the issues of the day as a senior leader of the main opposition Congress party despite his advancing age.

In August 2020, he told the BBC in a rare interview that India needed to take three steps "immediately" to stem the economic damage of the coronavirus pandemic, which had sent the country's economy into a recession.

The government needed to provide direct cash assistance to people, make capital available for businesses, and fix the financial sector, he said.

History will remember Singh for bringing India out of economic and nuclear isolation, although some historians may suggest he should have retired earlier.

"I honestly believe that history will be kinder to me than the contemporary media, or for that matter, the opposition parties in parliament," he told an interviewer in 2014.

Singh is survived by his wife and three daughters.