Deputy Prime Minister Heng Swee Keat's decision to leave his Tampines stronghold to lead the People's Action Party's five-member team in East Coast GRC was made to avoid a "succession gap" in uncertain times.

He said the move was decided on after discussions with Prime Minister Lee Hsien Loong.

Mr Heng, in a Facebook post, set out the reasons for springing one of the biggest surprises on the morning of Nomination Day yesterday.

He said: "The question was: Should I move to East Coast? I thought long and hard about it. After serving for almost a decade, I am very attached to Tampines and the people there."

Mr Heng, the PAP's first assistant secretary-general, said: "If I move, I can do my part in East Coast. We cannot afford a gap in East Coast in these uncertain times. We need a full team that can take care of the residents and position them to come out of this crisis stronger than before."

He added that Tampines GRC, where Environment and Water Resources Minister Masagos Zulkifli will replace him as anchor minister, has a "very good team".

Mr Heng said he and Mr Masagos have been working very closely on many projects and each time, Mr Masagos has delivered.

"In fact, we were in the midst of making many plans. I'm glad that many of the plans we did it together, so I'm confident that Masagos and his team will be able to deliver if our Tampines residents continue to give their support to our team," he added.

Mr Heng's replacement in his Tampines Central ward is Senior Minister of State for Trade and Industry Koh Poh Koon, who has moved to Tampines from Ang Mo Kio GRC.

At a PAP press conference yesterday, after the candidate slates were finalised, PM Lee was asked if he had dispatched Mr Heng to a constituency with fiercer competition to give him a chance to prove himself as a leader.

PM Lee said: "I don't think this is a manhood thing. He doesn't have to go there and prove that he is in the fiercest battle. It helps to have won a good fight, but he goes where it makes the most contribution to the overall campaign."

He was also asked if fielding Mr Heng in East Coast was a risky move. PM Lee said: "I think in an election, you fight to win, but you are never absolutely certain that you will win. We send somebody where we think he has a good chance of winning, and I'm quite sure that Swee Keat will put his all to make sure that he gets a good outcome in East Coast."

Mr Masagos said the Tampines team had its own "mini crisis" in 2016, when Mr Heng had a stroke and other members of the team had to help out. The team also took on some of Mr Heng's responsibilities after his recovery, when he was appointed Deputy Prime Minister and his workload increased.

Mr Masagos said: "I'm very honoured that he has entrusted us to run Tampines, and we will build on the good work he has left behind."

Mr Heng said he is leaving Tampines with "mixed feelings".

However, he said he is pleased that he and his team were able to deliver on many of the things promised to residents over the last two election cycles.

He said: "I feel comfortable that it is a team that can continue to serve Tampines residents very well.

In his Facebook post, Mr Heng said his new East Coast team is looking forward to working with residents to build on the residents' spirit of mutual care and support, and to emerge stronger from the crisis. "There is so much to learn, and I will work hard to learn it."

In East Coast GRC, Mr Heng will replace former manpower minister Lim Swee Say, who is retiring from politics.

In his post, Mr Heng paid tribute to Mr Lim for his contributions to Singapore and his work in his old constituency.

Mr Lim was best known for championing workers during his time as labour chief and manpower minister, and brought a "very balanced tripartite perspective to any discussion", he wrote.

Mr Heng recounted a dinner with Mr Lim 18 months ago, where the former minister, "in his usual way... spoke at length about everything he was passionate about".

This included Mr Lim's concerns and hopes for East Coast residents. The discussion, Mr Heng said, helped him to better understand the people in the constituency.

He said: "I look forward to meeting the residents of East Coast that our team speaks so fondly of. I hope to learn about your worries as well as your hopes. I hope to earn your confidence. If you entrust us with your vote, my team and I will do our best for you and for Singapore, to work with everyone to navigate Singapore out of this unprecedented crisis."

In a Facebook video message, Mr Lim, who first contested in East Coast in 2011, urged voters there to give his successor and his team their full support. He said he was hoping he would be succeeded by someone who was "young, smart and caring". When he learnt Mr Heng was coming to East Coast, he was "overjoyed".

Said Mr Lim: "All these years, I've always been impressed by his ability to see far, to think deep, to come up with bold and innovative policies. He not only knows a lot, but he cares a lot."