The Singapore prime minister's estranged brother has thrown his support behind a newly-formed opposition party in an intriguing twist ahead of elections expected to be held as soon as this year.

In Facebook comments made late Sunday, Lee Hsien Yang said the ruling People's Action Party (PAP) - led by his elder brother Lee Hsien Loong and founded by his father Lee Kuan Yew - had "lost its way".

His comments come amid a bitter feud between the siblings over their late father's house, and an election which some analysts say could be months away and must be held by early 2021.

"I wholeheartedly support the principles and values of the Progress Singapore Party," Hsien Yang said in the post.

"Today's PAP is no longer the PAP of my father. It has lost its way."

The PAP - which has ruled Singapore since its independence over half a century ago and never seen its vote share drop below 60% - did not respond to a request for comment.

The newly-formed Progress Singapore Party is led by Tan Cheng Bock, a former PAP lawmaker who shot to prominence by nearly defeating a candidate backed by Prime Minister Lee in the 2011 presidential race.

Lee, 67, has said he will step down as premier by the time he is 70, pitching the ruling party into a period of transition at a sensitive time.

Singapore's economy is faltering, there is unease over inequality, and social media has given critics of the government - both genuine and fake - a platform like never before.

It is unclear whether Hsien Yang plans to enter politics. Tan has said Hsien Yang would be welcome to join the PSP on their terms and not for any personal agenda.

Analysts were uncertain over what impact the PSP and Hsien Yang might have in upcoming elections.

Singapore is unlikely to experience a seismic political shift like the one seen last year in neighbouring Malaysia, where a party that had led every post-colonial government suffered an election defeat.

The PAP currently holds all but six of 89 elected seats in parliament.

The PSP may only further splinter the opposition vote, according to Garry Rodan, a professor at Australia's Murdoch University focused on Singapore politics.

But Michael Barr of Flinders University in Adelaide said if Hsien Yang does decide to stand in the election, a "Lee vs Lee" tussle could be a "game changer".