Prime Minister Najib Abdul Razak has fired an intercontinental salvo against his former mentor-turned-foe Dr Mahathir Mohamad.

Speaking to reporters in London, he commented on the former premier's remarks of preventing BN from winning in the upcoming by-elections, which is seen as a stab in the back.

Najib noted how during his 22-year tenure as prime minister, BN and Umno had helped Mahathir and his family reap enormous benefits.

"He and his family had derived a lot of benefits from his position as PM which was due to the strength of Umno and BN.

"So, when he told the people to reject the party which had done a lot, there were negative reactions against his stand because without BN and Umno, he would not have become prime minister or even a minister," he was quoted as saying by Bernama.

Najib was asked to comment on Mahathir's statement which called on opposition parties to be united against BN in the Kuala Kangsar and Sungai Besar by-elections on June 18.

The prime minister said many were of the view that Mahathir should not reject and campaign against the party that had contributed so much to the development of the nation.

Asked on whether he was disappointed with the matter, Najib said he had expected this of Mahathir.

Najib was speaking at a news conference last night at the end of his working visit to the United Kingdom.

Following this, the prime minister would be attending the two-day Russian-Asean summit.

The Kuala Kangsar and Sungai Besar parliamentary seat by-elections would be held following the demise of incumbents Wan Mohammad Khair-il Anuar Wan and Noriah Kasnon in a helicopter crash in Sarawak recently.

Meanwhile, Najib said BN was in the process of selecting the most suitable candidates for the by-elections.

The prime minister said the candidates to be fielded should be well accepted not only by the party, but also voters.

"At the moment, we are assessing suitable candidates to be fielded.

"We need to look into all aspects to ensure we have the right candidates who will be well received by both the party and voters," he told the same press conference.