Background: The COVID-19 pandemic has necessitated identifying individuals at higher risk of severe outcomes who should be shielded and provided with government support. We describe the initial results and validation of a web-based tool targeted at patients in the UK with inflammatory bowel disease (IBD) to self-stratify their risk according to a risk grid developed by an expert consensus body within gastroenterology.Methods: We designed a secure web-based survey, compliant with information governance, which was targeted directly at patients in the UK with IBD and promoted via social media. Patient-entered data were directly compared to data held by their local IBD specialist teams.Findings: Between 1st April and 3rd August 2020, we received responses from 34,078 participants with IBD from 176 UK trusts or health boards. Overall, based on these data, 25·9% of participants met the consensus criteria defined for shielding and a further 46·5% were in a moderate risk category. We assessed intra-rater reliability in 1,442 participants using the tool twice or more; most items had almost perfect agreement (kappa >0·80).We validated the patient-entered data against hospital-entered data and medical records for 2,862 patient datasets from ten hospitals. Weighted kappa was 0·59 (95% confidence interval 0·56 - 0·62). After manual resolution of discrepancies, kappa was 0·89 (95% CI 0·87 - 0·91). Of 966 patients identified as requiring shielding in the final dataset for these ten centres, 51·0% had been missed by the hospital-entered data, largely because of incomplete or discrepant information on comorbidity and current disease activity.Interpretation: We have demonstrated that patient-generated data can facilitate rapid risk stratification with respect to COVID-19 and compensate for deficiencies in hospital data. We have validated these data across repeat entry and have demonstrated their reliability compared to pre-existing secondary care data. These findings have important implications for public health and chronic disease management.Funding Statement: Galapagos Biotech Ltd; Biogen GmbH; Tillotts Pharma UK Ltd; Amgen LtdDeclaration of Interests: Author Vida Cairnes has received honoraria for speaking from Falk, Pharmacosmos, Abbvie and Tillots. I have received support to attend conferences fromPharmacosmos, Ferring, Takeda, Falk and Abbvie. I am in receipt of education funding from Crohn’s & Colitis UK. All other authors have nothing to declare. Ethics Approval Statement: The IBD Registry was the Data Controller for the COVID-19 UK IBD Risk Tool. Information Governance (IG) included the DPIA, Privacy Notices, approved information/content development for patients plus Information Sharing Agreements for hospital teams in order to allow the data to be shared. The lawful basis for data collection was under the COPI Notice issued for COVID-19.10 Information was freely provided by participants.