Ontario's top public health doctor told CP24 that Ontario labs detected 251 new COVID-19 cases in the past 24 hours, and Premier Doug Ford says people in Ottawa, Peel Region and Toronto should prepare for new restrictions.
Officials also confirmed four new deaths from infection, all in residents of long-term care homes.
Toronto reported 73 cases on Tuesday, down from 112 on Monday, while Peel reported 42, down from 71 on Monday.
"We'd still like it lower but let's see how it goes over the course of the week," Chief Medical Officer Dr. David Williams told CP24 after announcing Tuesday's number live on the air.
Ontario as a whole reported 313 cases on Monday, the highest daily count of infections reported since early June.
The high numbers have prompted calls for new restrictions from Brampton Mayor Patrick Brown and Toronto Mayor John Tory.
On Tuesday afternoon, Premier Doug Ford told reporters new targeted measures are coming.
"Over the next day or two we are going to be rolling out announcements for the regions that are affected," he said.
He said that unless Ontarians have been "living in a cave," they should be aware that large gatherings in their own homes are driving the recent spike in COVID-19 cases.
Provincial labs completed 27,664 tests in the past 24 hours, generating a positivity rate of 0.91 per cent, down from 1.06 per cent on Monday.
More than 24,000 test specimens remain under investigation.
Fifty-seven per cent of new cases were in people aged 39 or younger.
There are now 2,157 active cases of novel coronavirus infection in the province, up from 2,027 on Monday. Another 117 people recovered from infection.
Apart from Toronto and Peel, Ottawa reported 51 new cases, Durham reported 7 cases, York Region reported 22 and Halton reported 6 new cases.
Williams' remarks and release of Ontario's new caseload come as the largest school board in Ontario, Toronto District School Board, begins its school year.
But parents and the general public have seen Ontario's COVID-19 case counts increase steadily over the past two weeks, from less than 100 per day to more than 300 per day Monday.
The increase has sparked concerns about the onset of a "second wave" of the virus' spread.
"Clearly we are not starting this process on the right foot," UHN epidemiologist Dr. Isaac Bogoch said Tuesday. "The whole plan was to suppress cases as much as possible over the summer."
Bogoch said he expected case increases to begin later in the fall, mirroring what federal officials have said about their modelling.
Ontario's centrally-reported hospitalization data indicates 47 people are being treated for COVID-19 symptoms, but a count of data from local public health units on Tuesday found that at least 75 people were being treated in provincial hospitals on Tuesday, down from at least 77 on Monday.
Williams did not say why he thought cases were increasing so early in September only that private gatherings in homes that do not respect physical distancing are largely to blame.
He said that parents concerned about the possibility of major spread of the virus in schools needs to understand that the measures put in place in school will make a difference.
"We are not going to naively say there will be no cases, there will be some," he said.
"But there is a lot more infrastructure in place this time than in March to monitor schools."
He said 517 public health nurses have been hired since early August to assist schools with testing, tracing and outbreak management.
But he stressed that the first line of defence rests with parents monitoring their child for symptoms and not sending them to school.
When asked at what point he would order a second closure of schools, Williams said there is no definitive threshold.
"We're going to handle this in a very systematic way before we make any decisions to close down the schools like we did in March."