Health Minister Jonathan Coleman and Police Minister Judith Collins also announced their intentions to challenge for the top job, meaning supporters of Mr English will need to work hard to secure the numbers.
Mr English, who can use tomorrow's fiscal update to push his case for stability of the Government and economy, was anointed by Mr Key.
Mr English's supporters include Immigration Minister Michael Woodhouse, from Dunedin, Education Minister Hekia Parata and Primary Industries Minister Nathan Guy.
First-term MP Nuk Korako is also a supporter of Mr English and could be in line for a promotion should Mr English win.
Auckland Central MP Nikki Kaye returned to Parliament yesterday after dealing with breast cancer and expressed her support for Mr English.
Ms Kaye (36) intends standing again in Auckland Central. If her health remained clear, she would make an ideal deputy for Mr English.
It is likely Mr Woodhouse is doing the hard numbers work for Mr English (54), given his seniority in the Cabinet.
Mr Korako is an experienced member of Ngai Tahu enterprises and although his first Bill was derided as a time-waster, he will know how to count.
As Health Minister, Dr Coleman (50) sacked the Southern District Health Board. He has some strong supporters in his camp, although none of them has made their support public as yet.
Leader of the House Gerry Brownlee is reported to be a close friend of Dr Coleman and Mr Brownlee has played a pivotal role in the last two leadership contests.
Dr Coleman said he was standing because he sensed an appetite for change.
He represents the new face of National, coming into Parliament in 2005 compared with Mr English being an MP since 1990 as an original member of the ''brat pack'' which included Housing Minister Nick Smith, Roger Sowery and Tony Ryall.
Ms Collins (57) has been an MP since 2002 and said she had made the tough decision to stand for the leadership next Monday.
There are no early signs of who the three main contenders will pick as their deputies. Naming someone early can have advantages as well as disadvantages.
Climate Change Minister Paula Bennett, who is the fifth-ranked minister in Cabinet, remains coy about her intentions about the top job but is now seen as an ideal deputy.
The Otago Daily Times was told Mrs Bennett is being encouraged not to stand and split the caucus vote further.
Justice Minister Amy Adams was linked to Dr Coleman earlier yesterday and Ms Collins could get Transport Minister Simon Bridges interested in the job.
When Mr Key banished Ms Collins to the backbenches, she did not sit idle waiting for her chance to return to the Cabinet.
Instead, she worked with the backbench MPs, supporting their complaints about not being kept informed properly and having little to do.
The Police Minister can expect support from many of those MPs but it may not be enough to push her to the top of the list.
The leadership contest will go down to the wire late on Sunday and the vote on Monday will be a formality.
The contest has so far come down to two Auckland electorate MPs competing with a Wellington list MP with strong links to Southland and the South Island.
One of the three will probably withdraw late on Friday or on Saturday to leave the two main contenders to battle it out on Sunday.
An opinion poll taken within hours of Mr Key's resignation announcement shows Mr English is clear favourite for the top job.
The Fairfax-Nielsen poll was conducted on Monday evening and asked 1016 people who the next prime minister should be.
Of those with an opinion, 37% backed Mr English.
Steven Joyce was a distant second with 6%, followed by Judith Collins and Paula Bennett tied with 4% each.
No-one named Jonathan Coleman as their preferred option.
