Bill English's resignation National leader marks the end of one of the country's longest political careers.

The two-time National leader first entered Parliament in 1990.

Political editor Barry Soper says he's a determined, straight-shooting politician who will be sorely missed by many people.

"Taking National to it's biggest ever loss in 2002, he came back fighting fit as Finance Minister under John Key and then unfortunately for him, he couldn't pip Jacinda Ardern at the post."

But Soper says whoever replaces Bill English will face an uphill battle to win the next election.

READ MORE:

Andrew Dickens: Why I'd like to thank Bill English

Who could be National's next leader?

Bye Bill: English's political career with National

He earlier told Leighton Smith National may opt for a fresh face and change of direction for 2020, or they may choose to focus on winning in 2023.

"Whoever takes over the National Party would find it very difficult to win in 2020. Not a great deal is going to happen between now and the election, except for a baby being born."

Soper says English made a couple of mistakes in who he picked as his support staff, with deputy Paula Bennett seen as his "Achilles heel".

"Where English failed, and failed big time, was not with the electorate but with the coalition negotiations. He went into them with three people on the negoaition team who should never have been there if he wanted to curry favour with Winston Peters.

"One was Paula Bennett, the other was Steven Joyce and the third one was former Prime Ministerial Chief of Staff, Wayne Eagleson. Peters felt that they all were party to the [leaking of his pension overpayment]."

LISTEN ABOVE AS BARRY SOPER SPEAKS WITH ANDREW DICKENS