Tun Dr Mahathir Mohamad has pledged to support Pakatan Harapan in its campaign for the upcoming Sungai Besar by-election next month.

This was confirmed by former Batu Kawan UMNO deputy chief, Datuk Khairuddin Abu Hassan, in a talk last night.

Khairuddin said, the former premier had given him the green light to assist Pakatan Harapan candidates in campaigning for the by-election.

"Tun M has pledged and agreed to help me campaign for the by-election. This is the promise Tun M made," he said at a gathering with Parti Amanah Negara (Amanah) leaders in Sekinchan, Selangor.

READ: What exactly is Dr Mahathir's stand? - Hishammuddin

Khairuddin's speech answered the speculations about Dr Mahathir's support towards the Opposition in the June 18 by-election.

Earlier this month, Dr Mahathir had called for combined efforts to defeat Barisan Nasional (BN) at the two parliamentary by-elections, Sungai Besar and also Kuala Kangsar.

UMNO leaders subsequently lashed out at the former prime minister, saying he was betraying UMNO and its allies, which he had led for more that two decades.

UMNO secretary-general Datuk Seri Tengku Adnan Tengku Mansor had described Dr Mahathir's intention as 'to kill BN' when he called for the defeat of the ruling party in both by-elections.

Tengku Adnan had said, it seemed that Dr Mahathir had initially wanted to topple only Datuk Seri Najib Razak as UMNO president and BN chairman, but now he wants to kill BN altogether.

The two parliamentary seats, both won by BN in the 13th general election, were declared vacant following the death of their MPs in a helicopter crash in Sebuyau, Sarawak on May 5.

Both by-elections would be held simultaneously on June 18.