Helen Clark must explain to the public why she set up the Ingram inquiry into Taito Phillip Field when it was obviously designed to fail, says National Party Leader John Key.

He is responding to a report that the Labour MP may be charged with corruption and breaches of the Immigration Act.

"If corruption charges are laid, then Helen Clark has some serious explaining to do about why her inquiry was deliberately set up to fail," Mr Key says.

"It had no powers to call witnesses or obtain documents. It cost half a million dollars of taxpayers money and ended up as a virtual cover-up.

"The only thing it achieved was to prolong the Labour Government's one-vote majority for a few more months - in other words to buy time.

"Helen Clark has put the interests of her own party ahead of the public's expectations that MPs act within the law, and with integrity.

"This is another nail in the coffin of the dying Labour Government."