Japan's ruling party should not fight this year's election with Prime Minister Taro Aso at its helm, a former party No. 2 said on Thursday, underscoring the group's growing frustrations with the unpopular leader.

Aso's support ratings have tumbled ahead of an election that must be held by October, fuelling worries among lawmakers in the Liberal Democratic Party (LDP) that it could lose power after more than a half-century of almost unbroken rule.

"I don't think we can fight an election under Mr Aso," former LDP secretary-general Tsutomu Takebe told Reuters in an interview.

"He has lost the trust of the people. You can't run a government successfully without the trust of the people."

But Takebe, who served as the LDP's No. 2 under popular former prime minister Junichiro Koizumi, added that whether to step down or not would be up to Aso to decide, and that the leader should stay on until parliament passed the budget and related bills.

The budget is likely to be enacted by late March but related bills needed to implement the spending could take longer.