At the start of the COVID-19 pandemic, there was uncertainty surrounding the diagnosis, prevalence, phenotype, duration, and treatment of Long COVID. This study aimed to (A) describe the clinical phenotype of post-COVID symptomatology in children and young people (CYP) with laboratory-confirmed SARS-CoV-2 infection compared with test-negative controls, (B) produce an operational research definition of Long COVID in CYP, and (C) establish its prevalence in CYP.In total 219,175 CYP aged 11-17 years who had a positive (n=91,014) or negative (n=128,161) PCR test for SARS-CoV-2 between September 2020 and March 2021 in England were invited to participate. Test-positive and test-negative CYP were matched, at study invitation, on month of test, age, sex, and geographical region. 31,012 consenting CYP enrolled into the study at 3-, 6- or 12-months after their index-PCR test and, depending on when they enrolled, they were also invited to fill in follow-up questionnaires at 6-, 12-, and 24-months post index-test. The overall response rate was 14.1%, with retention across sweeps varying from 36.6% to 54.1%.A sub-study was set up in January 2022 when the Omicron variant was dominant. In the sub-study an additional 5,135 CYP who were PCR positive for the first time in January 2021 were invited, along with 4,507 who were reinfected during this period, and 5,157 who remained PCR-negative. 3,046 consenting CYP enrolled into the sub-study and filled in questionnaires at 0-, 3-, and 6- and 12-months after testing.The datasets include repeat self-reported information on CYP's physical and mental health over time, using validated scales. For the main sample, flexible survey weights have been developed to re-weight analyses to be nationally representative of CYP in England.Further information is available on the UCL Long COVID in Children and Young People (The CLoCk Study) webpages.Suitable data analysis softwareThe depositor provides these data in R format (.rda). Users are strongly advised to analyse them in R, as transfer to other formats may result in unforeseen issues. Long COVID in children and young people