The early years of his government were marked by liberalization of the economy and a subsequent economic boom, with India experiencing 7-9 percent annual economic growth. However, the past couple of years have been dogged by slowing growth and manufacturing, inflation, declining foreign investment and the tumbling value of the rupee. The Indian economy grew by 5.3 percent in 2013, according to government estimates, compared to 10.5 percent at its peak in 2010.
The country needs annual growth of at least 7 percent to "alleviate poverty and to create badly needed new jobs," Deepak Lalwani, director of Lalcap, which specializes in Indian investments, told CNBC.
"Singh came in on the back of very high hopes, based on his credentials for reforms in the 1990s, and in the early years, it seemed that the expectations were justified," Mark Williams, chief Asia economist at Capital Economics, told CNBC.
"In retrospect, the strong performance probably owed a great deal to reforms implemented under his predecessor."
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The rupee, which has plunged by 10 percent against the U.S. dollar in the past four months, is currently ranked among the "fragile five" emerging market currencies which are causing concern amongst investors. These countries: India; Indonesia; Brazil; Turkey and South Africa are linked by large current account deficits (where imports outweigh exports), elections this year, and vulnerability to the U.S. Federal Reserve reducing its asset-buying program.
"India is still under threat of its international credit status to be downgraded to "junk" - which if it materialized would be the first for a BRIC nation to face such ignominy," Lalwani warned.
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The economist and former governor of the Reserve Bank of India and Finance Minister had a long and distinguished career before becoming Prime Minister in 2004. His appointment was a surprise, as Congress Party leader Sonia Gandhi had been expected to lead the government after a Congress-led coalition won the election.
He urged Indians to vote in Gandhi's son Rahul during elections later this year. At the moment, pro-business Narendra Modi, currently chief minister of Gujarat, is leading in the polls – but Singh said on Friday it would be "disastrous" for India if he gained power.
One of the reasons for his criticism of Modi is the opposition politician's uneasy relationship with India's Muslims. Some blame Modi, a member of the Hindu nationalist BJP party, for failing to halt the anti-Muslim riots in Gujarat which resulted in over 1,000 people being killed in 2002.
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While in power, Singh was more of a technocrat – an expert who is not a career politician or, in some cases, even a member of parliament – than a traditional political figure. This was initially seen as an advantage, in a country notorious for political corruption.
"I have tried to serve this country with upmost dedication, commitment and integrity…I have never used this office to enrich or reward my friends or my relatives." he told reporters at the conference announcing his resignation on Friday.
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His biggest mistake, according to Williams, was failing to continue the program of reform and make India's notorious red tape easier for businesses to negotiate. His government focused more on redistribution of India's new wealth via job creation schemes.
"The lesson of the last few years is that, unless you continue these reforms, sustainable growth won't be built," he said.
- By CNBC's Catherine Boyle. Twitter: @cboylecnbc.