Former premier Dr Mahathir Mohamad has called on the opposition and the public to cast aside their identities as party members in the upcoming twin by-elections to ensure Prime Minister Najib Abdul Razak's defeat.

Speaking after a meeting with several opposition MPs in Putrajaya, Mahathir said this was because some of Najib's policies, such as the goods and services tax, have burdened all regardless of political allegiances.

"We are agreed that in these by-elections we must behave as though we don't belong to any party.