Singapore must start planning for a population that could possibly hit 10 million, Liu Thai Ker, the man often credited as the architect of modern Singapore, told CNBC.

The bold number suggested by Liu, who served as the chief executive of the Housing Board from 1979-1989 and then as CEO and chief planner of the Urban Development Authority from 1989-1992, is nearly double the current 5.3 million population and significantly higher than the 6.9 million figure proposed by the Singapore government in its 2013 Population White Paper.

In the white paper, the government described its vision of raising the country's population by as much as 30 percent in the next two decades to ensure the economy remains dynamic. However, the move sparked strong objections amid rising discontent in the land-scarce nation over soaring housing costs and an influx of immigrants.

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But Liu stands by his theory, saying that population growth is pivotal to Singapore's future.

"One fundamental thing about urban planning is, don't try to stop or control or curb population growth," Liu, who is now chairman at the Centre for Liveable Cities and senior director at RSP Architects Planners and Engineering, said.

"We should allow Singapore to grow and plan for a much bigger population… like 10 million people. We should ask ourselves: How long do we want Singapore to remain as a sovereign country? Even at 10 million people and assuming a population growth rate of 1 percent, we will only last slightly over 100 years and that's not a long time," he added.

The country, which is battling worrying demographic changes, also needs immigrants to keep its economic engines running. With a fertility rate of only 1.2, far below the replacement rate of 2.1 and one of the lowest in the world, an ageing population would lead to profound problems for Singapore, the country's Prime Minister Lee Hsien Loong said at a conference earlier this month.

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Liu agrees: "Being such a tiny place, there is a propensity toward homogeneity of ideas and concepts. For us to nurture a creative society, we need people from outside. In fact, one of the reasons why Singapore could succeed was because we were a heterogeneous society at the beginning, with people from all over the world."

But even as population numbers accelerate, Singapore's achievements in urban development and innovation must be maintained.

"We must continue to keep the city green and attractive for businesses, as well as good talent to come," Liu told CNBC.

Lessons from Singapore

Apart from being well-known as a 'garden city' where flora and fauna is weaved into the urban fabric, the stability and efficiency of Singapore's urban infrastructure serves as a role model for many developing nations.

To emulate the success of the Southeast Asian city-state, governments in these developing countries will need to take the lead, according to 77-year-old Liu.

"It may not sound democratic in a Western sense but in Asia or even Africa, the government must play a big role when there's a great need for development. Because if you leave it to the businesses or private sector, they will inevitably focus more on the business side of things."

For that reason, it is imperative that government leaders educate themselves on urban development, the architect-planner added.

"Leaders must be humble enough to learn what makes a good city. Mr Lee understood what made a good city from his days in Cambridge, but he spent all his life learning from urban success stories," said Liu, referring to Singapore's founding father Lee Kuan Yew.

"Leaders must also be ruthlessly rational... and I often explain to foreign dignitaries that the highest authority in Singapore is something called the truth. The Prime Minister and President will listen [even if it was] a lowly civil servant who said the truth. That, to me, is an important aspect of Singapore's success story."

Liu retold the story of Lee 's decision to construct low-cost flats in high-rise buildings - known as HDBs - even though such high-density housing was condemned by experts in the 1960s. Towering skyscrapers have since become a symbol of the nation's successful public housing strategy and urban landscape.

"We must subscribe to 'clarity equals courage.' It is not good enough to have courage and charge ahead blindly. it is also not good enough to just follow the world. You need to think what your city needs and have the courage to move ahead even [if it is] against the world's trends," he added.

Lee Kuan Yew was Singapore's first and longest-serving prime minister, who oversaw Singapore's transformation from a sleepy British colonial outpost into a global metropolis within a single generation. He died on March 23 at the age of 91.

— Reporting by Christine Tan | Written by See Kit Tang