Prime Minister Najib Abdul Razak has urged federal opposition pact Pakatan Harapan to stop misleading the people with its promise to abolish the goods and services tax (GST) and replace it with the older sales and services tax (SST) system.

In a blog post, Najib said this was because the SST is an outdated system that could indirectly burden the people, unlike the GST which is more transparent.

"I want the opposition, if they are truly sincere to abolish the GST, not to lie to the people by saying that they will replace it with the outdated SST system," said Najib, who is also the finance minister.

He also said that abolishing the GST and reverting to the SST would be akin to "changing a round wheel to a square wheel".

"Introduction of the SST is not only a regressive move, but would indirectly burden the people.

"Tun Mahathir knows this and had in the past acknowledged that the GST is better than the SST. This is because his own administration had proposed for Malaysia to upgrade to the GST system," he said in hitting out at former prime minister Dr Mahathir Mohamad.

Najib further accused Mahathir of doing another "U-turn" or change in stand on the implementation of the GST, in order to seek political mileage from the people.

"Just to beg for votes, Mahathir wants to show as if the opposition will abolish the GST, but on the other hand (they) replace it with the SST where the rate is not informed to consumers.

"Isn't this a lie? Under the SST system, there is no need for traders to state the amount of tax in the receipt," said Najib, who pointed out that consumers risk being cheated by traders who imposed a high tax rate.

This, he said, was unlike the GST with its fixed rate of six percent.

By reverting to the SST, Najib also claimed Mahathir has been forced to concede that income from the GST has helped the government to cover an RM40 billion shortfall in oil revenue.

Najib went on to challenge Harapan to abolish the GST without reverting to the SST, and to come up with other ways to make up for the shortfall in oil revenue.

Harapan has vowed that it would scrap the GST within 100 days of taking over Putrajaya in the next general election, the 14th for the country.

Mahathir, meanwhile, had in September revealed that one of the ways being considered was to replace the GST with its predecessor, the SST.