Warehouse workers keeping "essential supplies moving" should be given greater protection, a mayor has said.
Dan Jarvis called for stronger social distancing measures for South Yorkshire's 21,000 warehouse workers.
Concerns have been raised over conditions at sites in the region used by clothing retailers ASOS and Pretty Little Thing.
The Labour MP said a "number of businesses" in the region "continue to operate unsafe working practices".
Mr Jarvis, mayor of the Sheffield City Region, said: "Coronavirus is going to dominate our way of life for the weeks and months ahead, so for everyone's safety, it's vital we look after workers who are making sure communities have the supplies they need to get through the crisis.
"The government must provide clear and unequivocal guidance about the required social distancing practices. They must ensure they are implemented and take tough action where rules are broken."
A number of retailers have warehouses in South Yorkshire where food and other goods are stored and sorted before being distributed around the country.
Last month, Pretty Little Thing said it had "implemented stringent hygiene and self-distancing measures" amid claims its Sheffield warehouse was "a breeding ground for Covid-19".
While earlier this week, ASOS said it "totally refutes" claims from a workers' union that it was risking the safety of employees at its Barnsley warehouse by not enforcing social distancing measures.
Mr Jarvis, the MP for Barnsley Central, said he had been working with trade unions and local authorities to protect workers' safety, according to the Local Democracy Reporting Service.
He has written to the Secretary of State for Business, Alok Sharma, asking for clearer guidance and enforcement of social distancing rules where businesses continue to operate.
"South Yorkshire's warehouse and distribution workers are keeping our supermarkets stocked, our businesses supplied and our communities connected during the coronavirus," he said.
"Their hard work is overlooked and undervalued, but during this challenging time, we must appreciate their selfless efforts, which are helping countless others."