Carers Trust said figures gathered following a survey give an "incredibly worrying sense of the scale of this issue for the first time in a decade", adding that they "blow all previous figures out of the water".
Official figures from the last census in 2011 put the number of young carers in England aged between five and 17 at 166,000, although this has long been thought of as an underestimate.
Research published this week calculates there could be as many as 800,000 young carers of secondary school age.
The research, carried out by Nottingham University and BBC News, involved 925 children across England, aged 11 and 12 and 14 to 15.
Extrapolated across England the results show that of the 800,000 young people of secondary school age carrying out care, 250,000 of these are undertaking a high level of care, with 73,000 taking on the highest level of care.
The Carers Trust said the statistics act as a "monumental wake-up call for us all to take responsibility for these vulnerable children".
"Whilst these new figures are shocking, we are not at all surprised. A lack of government commitment to making sure these vulnerable children are routinely identified in school and supported means young carers are slipping through the net," said Carers Trust chief executive Giles Meyer.
"Carers Trust is today calling on the government to take urgent action to make sure that the rights they enshrined in law to protect all young carers are actually being delivered by local authorities, rather than the patchy, ad-hoc support services currently out there."
Carers Trust says that being a carer can have a devastating impact on young people's lives, affecting their school attendance, academic achievement, future life chances and mental and physical health.
A survey of 350 young carers by the charity found that just under half (48 per cent) said their caring role made them feel stressed and 44 per cent said it made them feel tired.
On average young carers miss or have to cut short 48 school days a year and a quarter says they have experienced bullying due to their caring role, The Carer Trust adds.
In June the government pledged to take action to ensure young carers are identified and supported earlier. Its two-year Carers Action Plan for 2018-20 said it is linking up with the Carers Trust to develop a "young carers identification project" to help children's professionals better spot children carrying out caring duties.
This action plan is separate to a delayed cross-government Carers Strategy, which was due to be published last year but will now be released alongside a forthcoming social care strategy.
A Department for Education spokeswoman said: "Young carers make an invaluable contribution in looking after their loved ones. That's why we changed the law to improve how we identify and support them and their families, and we continue to work with local authorities, schools and voluntary partners to this end.
"Our cross-government Carers Action Plan sets out measures to support young carers in every aspect of their lives over the next two years and our forthcoming green paper will look at long-term sustainable solutions for the social care system."