BACKGROUND: Diagnostic timeliness in cancer patients is important for clinical outcomes and patient satisfaction but, to-date, continuous monitoring of diagnostic intervals in nationwide incident cohorts has been impossible in England. METHODS: We developed a new methodology for measuring the secondary care diagnostic interval (SCDI - first relevant secondary care contact to diagnosis) using linked cancer registration and healthcare utilisation data. Using this method, we subsequently examined diagnostic timeliness in colorectal and lung cancer patients (2014-15) by socio-demographic characteristics, diagnostic route and stage at diagnosis. RESULTS: The approach assigned SCDIs to 94.4% of all incident colorectal cancer cases [median length (90th centile) of 25 (104) days] and 95.3% of lung cancer cases [36 (144) days]. Advanced stage patients had shorter intervals (median, colorectal: stage 1 vs 4 - 34 vs 19 days; lung stage 1&2 vs 3B&4 - 70 vs 27 days). Routinely referred patients had the longest (colorectal: 61, lung: 69 days) and emergency presenters the shortest intervals (colorectal: 3, lung: 14 days). Comorbidities and additional diagnostic tests were also associated with longer intervals. CONCLUSION: This new method can enable repeatable nationwide measurement of cancer diagnostic timeliness in England and identifies actionable variation to inform early diagnosis interventions and target future research.