Mahathir Mohamad has rejoined the United Malays National Organization (UMNO)

Mahathir and wife handing over the application forms to the new Prime Minister Najib

PUTRAJAYA, Malaysia (Reuters) - Former Malaysian premier Mahathir Mohamad has rejoined the United Malays National Organization (UMNO) in what may be a move designed to boost the ruling coalition's chances in by-elections next week.

Mahathir, who ruled for 22 years until 2003, submitted his application to rejoin the party on Saturday. It was accepted by new Prime Minister Najib Razak, a day after he took over the Southeast Asian country.

Mahathir still wields some influence within (UMNO), which he quit in a huff last year after months of criticism against the policies of his successor, Abdullah Ahmad Badawi.

He had said he would return only if Abdullah resigned.

Mahathir handpicked Abdullah to succeed him but later became his staunch critic, with some analysts saying Mahathir's criticism added to the pressure that forced Abdullah to leave office earlier than he had planned and allow Najib to take over.

Najib said Mahathir's return would "encourage UMNO and help to rebuild the party."

The timing of Mahathir's return may have been designed to boost the chances of the National Front coalition winning in three key by-elections next week.

"Mahathir will have a strong influence in the sense that he was prime minister for a very, very long time and is seen by UMNO as a mentor of sorts," Ooi Kee Beng, a fellow at the Institute of Southeast Asian Studies in Singapore, told Reuters.

"But one wonders too if this was all planned, that he would join just before the by-elections to give Kedah a boost. But while Mahathir still has a strong influence over some of the Malays, he is disliked by some quarters."

The National Front will have to face off against the opposition, led by former deputy prime minister Anwar Ibrahim, in a parliamentary by-election in Perak and in two state by-elections in Kedah and Sarawak on April 7.

"The image of UMNO has suffered somewhat over the last few years," Mahathir told reporters in the administrative capital Putrajaya.

"The first thing to be done now is to clean up UMNO."

Abdullah led the National Front to its worst election result in 2008 polls, as voters rebuked the government for its failure to deliver on promises to tackle civil service corruption and boost the economy.

"I was critical of UMNO whenever it goes wrong. If it doesn't go wrong, I don't see why I should be critical," Mahathir said, when asked if he would also be critical of Najib's performance.

"I think under the leadership of Najib, I don't think he will stray from the old path, which was set by his father, not me."

Najib's father, Abdul Razak Hussein, was Malaysia's second prime minister.

(Writing by Liau Y-Sing, Editing by Dean Yates)