Health chiefs in the West Midlands and Staffordshire issued the warning as they called on people to avoid being "careless" this weekend.
It comes as lockdown measures were eased earlier this week – allowing groups of up to six, or two households, to meet up outside.
Katie Spence, deputy director at Public Health England West Midlands, said: "West Midlands used to be the region with the highest case rates in the UK. Thanks to the community working hard we are doing much better.
"But I am concerned that with the freedoms given to us this week, especially with socialising over the Easter bank holiday, rates could go up again.
"We need people to stick to the rules, otherwise the roadmap is at risk. Most people respect the rules and think about the health and safety of their loved ones and I thank you all for that."
Ms Spence urged people to meet friends or family outdoors – in line with the guidance – and where it will be safer and called on people to remain as cautious as possible.
Staffordshire County Council's director of health and care, Dr Richard Harling, said everyone "cannot afford" to let their guard down and relax in the fight against the virus.
Dr Harling said: "There has been a great deal of hard work and sacrifice in the last 12 months to get to where we are today and it would be terrible to undo all that by becoming careless now.
"It’s up to us all to follow the ‘hands, face, space’ guidance and get tested regularly. That is the way to keep the virus under control and give us the best chance of getting our freedoms back."
Meanwhile, Staffordshire Police called on people to follow the rules and said patrols would be stepped up at places where they expect higher footfall over the weekend.
Chief Superintendent Jennie Mattinson, head of neighbourhood and partnerships, said: "We have seen the vast majority of residents across the county stick to the rules and do their bit in reducing the number of Covid-19 cases. By following the national restrictions people are not only keeping themselves safe but protecting the health and well-being of those around them.
"It has been a challenging 12 months for everyone and for the first time in a while we can begin looking forward to our sacrifices paying off and normality slowing returning to the county.
"But it is vital that we don’t put all our efforts at risk. We must continue to adhere to the rules and our officers will be out explaining these and encouraging everyone to play their part by following them, and taking enforcement where necessary. The Government has set out a roadmap and it is so important everyone plays their part by following the rules at each stage.
"We will have patrols in areas we anticipate a higher footfall and officers will be providing suitable advice. While people should quite rightly see this is as the first stage of normality slowly returning and enjoy themselves in a safe way, the rules are still very clear."