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National had decided that it would not introduce legislation that depended on the Epsom MP's vote, but didn't pressure him to resign, Mr Key said.
That means the Government does not have enough votes to progress the controversial Employment Relations Amendment Bill in this term of Parliament.
The PM could not think of any other bills that would have depended on Banks.
Leader of the House Gerry Brownlee said that with 12 more hours of the Budget debate and 16 valedictory speeches to get through, there was little time for much other legislation between now and when the House rises next month.
Without Banks, National will still have 63 votes on confidence and supply issues in the 120-seat House.
Unionists will celebrate the fact that the employment bill does not have the numbers to progress this term. It abolishes the duty of good faith on a union and employer to conclude a collective agreement, relaxes rules on meal and tea breaks and lets employers dock pay for partial strikes.
Banks last night announced he will resign next Friday, after being found guilty of filing a false donations return in respect of two $25,000 cheques from Kim Dotcom's Megastuff company for the 2010 Super City mayoral election.
"I have given my heart and soul over four decades to making a worthwhile contribution to this country," Banks said. "I have always endeavoured to do the right thing. Consequently I am deeply saddened at this turn of events."
Banks was convinced he was going to win the High Court case and friends told the Herald he had taken the verdict hard.
Act leader Jamie Whyte said the party had a lot to thank Banks for: he won the Epsom seat in 2011, had established partnership schools which was important to Act and - while he was more conservative than Act members - voted as an Act MP rather than according to his personal views on some occasions, such as in favour of the gay-marriage law.
Banks is due to be sentenced on August 1, when the judge will also say whether he will convict him. A conviction would have forced the MP from Parliament anyway.
What appears to have clinched his resignation is advice that in such an event, Parliament would have had to be recalled to vote against holding a byelection so close to the September 20 general election.
Dr Whyte spoke to Banks on Saturday and expressed his preference for him to resign.
Mr Key's chief of staff, Wayne Eagleson, also spoke to him, but the PM said National wasn't "putting massive pressure on him".
National needs the support of 75 per cent of the House to avoid holding a costly byelection, so effectively it needs Labour's backing.
Labour leader David Cunliffe said last night his initial view was that the public would not thank Labour for the expense of a byelection. But he wanted to talk to caucus colleagues as well as other Opposition parties.
Mr Cunliffe criticised Mr Key for describing Banks as an "honest" man after the verdict on Thursday.
"This Government's mandate has rested on a man found guilty of a serious electoral offence; on [United First MP] Peter Dunne, stripped of his ministerial warrants after he lost the trust of the Prime Minister having allegedly leaked a classified [Government Communications Security Bureau] report; and on a Maori Party cravenly propping up the Government."