According to data from Brighton and Hove City Council, the steep rise is being driven by dramatic rate increases in the 15-29 age group.
As of June 9, the rate was 303 per 100,000 residents. This is compared to a week previously where the rate was 89.6.
More up-to-date and specific data from the government’s website is available up to June 16.
As of last Wednesday, the rate in people aged between 20 to 24 was 435.
On the same day, the rate in teenagers aged 15 to 19 was 413.
The increases have been blamed on the new Delta variant, which is spreading faster and is more transmissible than the previous variant.
A council spokesman said: “We continue to be in close contact with schools and have given them updated guidance following the announcement from Government, to continue to be vigilant to keep the risk of transmission in schools down.
“This includes a regular review of existing measures, maintaining the integrity of bubbles and having robust risk assessments for upcoming transition and end of year events, holding them virtually wherever possible.”
The council says that it is working to encourage vaccination within the younger age groups.
The current data shows the rates are not rising steeply in older, vaccinated groups.
The spokesman added: “We are also working with our universities and young adults in the city, offering regular free testing, support for self-isolation to those that need it and encouraging everyone above the age of 18 to book their vaccination as soon as they can to reduce the effects of Covid and reduce transmission.
“So far the vaccination take up amongst this age group is looking very promising.”