Conservative MPs will decide whether to remove the party leader from office amid a backlash to the Partygate scandal.
The secret ballot has been triggered by scores of Tory MPs writing to party bosses calling on the PM to quit.
But Paymaster General Mr Ellis tweeted: “The Prime Minister continues to have my 100 percent support.
“He got Brexit done; he led the country through the pandemic and he has led the world in supporting Ukraine.
"We owe it to our constituents to focus on delivering to make lives better.”
Anger among Tory backbenchers has been rising since Sue Gray published her official report into lockdown parties in No 10 last month.
Dame Andrea Leadsom was the first county MP to break ranks and criticise the PM last week admitting he had been guilty of “unacceptable failings of leadership that cannot be tolerated.”
In a statement to South Northamptonshire voters on Tuesday (May 31), the former business secretary stopped short of calling for the PM’s resignation or a leadership contest.
But Dame Andrea warned: “Each of my Conservative MP colleagues and I must now decide individually on what is the right course of action that will restore confidence in our government.”
■ Click HERE to read the Sue Gray report in full
Mr Ellis was one of a number of cabinet members rallying around Mr Johnson ahead of the vote.
Chief Whip, Daventry MP Chris Heaton-Harris did not offer his own backing but retweeted messages of support from colleagues Liz Truss and James Cleverly.
The no-confidence vote was announced earlier by Sir Graham Brady, who chairs the 1922 backbench committee that organises such contests.
He confirmed he had informed the PM on Sunday that the required 15 percent threshold for a vote — 54 MPs — had been hit.
At least 180 Tory MPs need to vote against Mr Johnson to oust him from office.
The vote will take place in the House of Commons between 6pm and 8pm with the result expected to be revealed shortly after.