The Reform Party (RP) will not contest West Coast GRC, to avoid a three-cornered fight with the People's Action Party (PAP) and the Progress Singapore Party (PSP).

RP secretary-general Kenneth Jeyaretnam announced the decision at about 1.30am yesterday in a Facebook post, just hours after Parliament was dissolved and the Writ of Election issued.

The move likely leaves PSP to take on the incumbent PAP team in a head-to-head fight for West Coast.

On June 15, Mr Jeyaretnam unveiled six candidates, half of whom had been part of RP's 2015 team in West Coast, raising expectations of a three-cornered fight.

But yesterday, he said RP had been in "long talks" with PSP chief Tan Cheng Bock, and decided the "greater cause" at the moment was opposition unity and contesting where everyone has the greatest chance of winning seats.

"We also recognise that Dr Tan has a tough fight on his hands despite it being his old stomping ground and we do not wish to hinder his progress," he said.

Dr Tan thanked RP for not contesting West Coast.

"We all understand the implications of a three-cornered fight, so we are trying to put our differences aside so that we can contest with the PAP one on one," he told reporters yesterday morning.

The PAP defeated RP in West Coast in 2015 and 2011, and has won every election there since the constituency was formed in 1997.

But Dr Tan also often speaks of West Coast as his "home" ground.

It contains the former single seat Ayer Rajah, where he was the PAP's MP for 26 years until it was absorbed into the GRC in 2006.

Without revealing specific areas, Mr Jeyaretnam said PSP will withdraw from "various grounds" in return for West Coast. He said PSP also offered to cede Nee Soon GRC, but RP decided not to contest it.

Last night, a PSP spokesman confirmed that the party will still be contesting Nee Soon GRC, among nine constituencies announced by the party on Tuesday. When announcing the list of constituencies, Dr Tan had said it could still change.

"Of course, on Nomination Day, things change. Politics is very fluid, so we will study all these places more carefully," he said.

RP and PSP still have overlapping claims in Yio Chu Kang SMC. When asked if PSP might pull out to avoid a multi-cornered fight, the spokesman said that as of now, PSP still intends to contest Yio Chu Kang.

Party sources told The Straits Times that the candidate for Yio Chu Kang has already been decided, and is among the dozen unveiled by PSP so far.

Mr Jeyaretnam also said RP will contest in Radin Mas SMC and Ang Mo Kio GRC.

RP's decision to cede West Coast also prompted other concessions.

In a Facebook post at about 2.30am yesterday, the People's Power Party said it welcomed the RP's decision, adding that it "will be most willing to take a step back" and not field a candidate in Radin Mas, so that RP can focus on the ward.

Radin Mas contains the former Anson SMC - which was helmed by Mr Jeyaretnam's late father, veteran opposition leader and RP founder J. B. Jeyaretnam.