Pakatan Harapan chairperson Dr Mahathir Mohamad fired a salvo at Muhammad Muhammad Taib within hours of the latter rejoining Umno, raising an incident involving a bag of cash which led to his resignation as Selangor menteri besar in 1997.

"I still remember Mat Taib brought a bag full of cash and was arrested in Australia.

"He gave the excuse that he could not understand English which was why he broke the law... But I knew that he knows English.

"Where the money came from is not known, but there was a lot," he said in a video response uploaded on Parti Pribumi Bersatu Malaysia's (Bersatu) Facebook page.

Mahathir was referring to an incident where Muhammad, better known as Mat Taib, was arrested at Brisbane airport carrying Singapore, Malaysia and New Zealand bank notes worth US$1 million, based on the currency at that time.

Muhammad was subsequently charged for failing to declare that he was carrying more than A$5,000 to Australian immigration, but was later acquitted.

He had pleaded that he could not understand English and therefore could not comprehend Australian regulations.

Mahathir, who is also Bersatu chairperson, also cheekily suggested that bags carried by Muhammad should be checked for money.

Prime Minister Najib Abdul Razak today summoned Umno leaders from across the country to Kuala Lumpur to announce Muhammad's re-entry into Umno.

Muhammad had joined PAS in 2013 and later PKR in 2015.

Mahathir sarcastically congratulated Muhammad but questioned Najib's treatment of the retired menteri besar.

"I am curious why Najib (and his deputy) Ahmad Zahid Hamidi would take Mat Taib's entry (into Umno) as a major event that they would call a special press conference," he said.

Ahmad Zahid had to abandon his programmes in Perlis while International Trade and Industry Deputy Minister Ahmad Maslan had flown in from Sabah to Kuala Lumpur for the announcement.

Opposition leaders had braced for a major coup by Najib but later mocked the press conference after finding out that it was about Muhammad rejoining Umno.

Mahathir likened Muhammad to a "frog" and said he was happy that the latter had gone back to Umno.

"He behaves like a frog, from Umno, he jumped to PAS. From PAS, he jumped to PKR. In PKR, he expressed support for me in Bersatu.

"Now, his legs are still strong, so he jumped (to Umno).

"I congratulate Umno, the more people like Mat Taib they have, it will guarantee Umno's loss in the general election," he said.