A network of community hubs is being set up across the UK to deliver food to 1.5 million people with serious medical conditions who have been told to remain indoors for 12 weeks because of coronavirus.

Army planners have been drafted in to help organise the support for people with conditions like cancer or respiratory problems, who will receive letters over the coming days asking them to avoid all social contact for three months under a “shielding” plan to protect them from the virus.

The announcement came as Boris Johnson warned the public to take social distancing advice “seriously” when going to parks during the outbreak, warning that otherwise they put other people’s lives at risk.

Speaking at the daily Downing Street press conference, Mr Johnson warned that the government will have to bring forward “tougher measures” to protect the UK from the spread of coronavirus if people fail to act “responsibly” and observe the requirement to keep two metres away from one another when outdoors.

Standing alongside the prime minister, Robert Jenrick, the communities secretary, said free parcels of groceries would be delivered to the most vulnerable, adding: “Nobody needs to worry about getting the food and essential items that they will need.”

Mr Jenrick said those who are “shielding” and do not have family and friends living nearby will be able to get vital medicines and food from community hubs. He said the scheme will require a “major national effort” in a very short period of time.

He said: “Medicines will be delivered by community pharmacies. Groceries and essential household items will be delivered by local councils and food distributors, working with supermarkets, to ensure that nobody needs to worry about getting the food and essential items that they’ll need.

“These parcels will be left on the doorstep.

“The government, the food industry community pharmacists, local councils and emergency services are working round the clock to get this scheme off the ground, members of the armed forces are already supporting this effort, including some of the finest military planners in the world.”

Those using the community hubs to receive care parcels would be able to do so free-of-charge, with the Treasury footing the bill, Mr Jenrick confirmed.

He added: “If over time we’re able to refine it into a more sophisticated offer which is more tailored to individuals’ needs – and we’re working with supermarkets to see if that is possible in time – then it may be a different arrangement.

“But to begin with it’s going to be free to ensure that the people who really need it get the supplies as soon as possible.”

Mr Jenrick said he did not underestimate how tough the shielding period would be for those affected, but sought to reassure people that the government would make sure they had what they needed.

Mr Johnson said that the strategy would “do more than any other single measure that we are setting out to save life … to reduce infection and to slow the spread of the disease”.

On Sunday, NHS England announced that one of the victims was just 18 years old.