Amanah chief Mohamad Sabu said the party has always known that PAS was working with BN, even before the 14th national polls in May last year.

Therefore, its alliance with BN, which some PAS leaders have denied, is no longer a secret, said Mohamad, better known as Mat Sabu.

"They (PAS) used to work underground, now they are working with BN above ground," he said.

"This is indeed a betrayal of its members. But in Cameron Highlands, its members will now be asked to vote for BN," Mat Sabu told reporters on the sidelines of his son's wedding celebrations in Guar Petai, Tasek Gelugor, Penang.

Mat Sabu, a former PAS deputy chief, left the Islamic party to form Amanah.

However, he said "winning in Cameron Highlands is tough; we have to work very hard".

He was commenting on Umno secretary-general Annuar Musa's statement that at least 15-20 PAS leaders would canvass for votes in the parliamentary constituency, where Malay-Muslims form the majority of the electorate.

Mat Sabu said the battle is made even tougher as BN candidate Ramli Mohd Nor is "good" as he comes from the community.

"But we are more worried that our supporters from outstation will not be returning to vote on polling day due to difficulties in getting leave," said the defence minister.

Asked to comment about the low turnouts during Pakatan Harapan ceramah in Cameron Highlands in which he and Finance Minister Lim Guan Eng attended, Mat Sabu said "it is normal, this is a by-election".

The Cameron Highlands by-election will see Pakatan Harapan attempting to wrest the seat from BN since the May 9 polls.

BN has fielded Ramli, who is not a member of any BN coalition party, but being a local boy and a former top cop, is seen as a winnable candidate.

He may draw his support from the Malay and Orang Asli, who collectively make up slightly over half the electorate.

DAP has re-fielded M Manogaran, a lawyer, who is making his third attempt at winning the Cameron Highlands seat.

BN had won the parliamentary seat by a sliver in 2013 and 2018.

Polling is on Jan 26 after a 14-day campaign.