Speaking during the government's daily press coronavirus conference, Prime Minister Boris Johnson praised the public for having responded to the call for volunteers made by health secretary Matt Hancock on Tuesday afternoon so enthusiastically.
"Coupled with following all the advice and guidance volunteering like this is a great way to reduce the strain on our NHS so that they can do their bit, so let's come together in the spirit of co-operation".
The 405,000 figure is up from 170,000 announced just hours before.
Boris Johnson announced that 405,000 people have signed up to be NHS volunteers.
"I owe you a debt of thanks", he added.
The NHS are seeking volunteers to help battle coronavirus.
"This volunteering scheme is open to over 18's as long as they're fit, healthy and showing no symptoms of COVID-19, and the NHS is making sure that your safety isn't compromised because the majority of these tasks can be done whilst following social distancing guidance.
We are proud to support the NHS at this important moment and we are certain many thousands of people will selflessly step up to play their part", she said per The Times.
"The NHS is always there for you - now is your time to be there for us too".
He said the equivalent of the population of Coventry had applied to help in just 24 hours.
He told BBC Radio 4's Today Programme that there would be a variety of jobs available, including parcel delivery and driving.
Anyone in good health and not in a vulnerable category can sign up to help as a Check in and Chat Volunteer to phone those at risk of severe loneliness, a Community Response Volunteer to collect shopping and other essential supplies for someone who is self-isolating, or a NHS Transport Volunteer to transport equipment, medication and supplies between NHS sites.