BACKGROUND: The COVID-19 pandemic resulted in the widespread disruption of cancer health provision services across the entirety of the cancer care pathway in the UK, from screening to treatment. The potential long-term health implications, including increased mortality for individuals who missed diagnoses or appointments, are concerning. However, the precise impact of lockdown policies on national cancer health service provision across diagnostic groups is understudied. We aimed to systematically evaluate changes in patterns of attendance for groups of individuals diagnosed with cancer, including the changes in attendance volume and consultation rates, stratified by both time-based exposures and by patient-based exposures and to better understand the impact of such changes on cancer-specific mortality. METHODS: In this retrospective, cross-sectional, phase-by-phase time-series analysis, by using primary care records linked to hospitals and the death registry from Jan 1, 1998, to June 17, 2021, we conducted descriptive analyses to quantify attendance changes for groups stratified by patient-based exposures (Index of Multiple Deprivation, ethnicity, age, comorbidity count, practice region, diagnosis time, and cancer subtype) across different phases of the COVID-19 pandemic in England, UK. In this study, we defined the phases of the COVID-19 pandemic as: pre-pandemic period (Jan 1, 2018, to March 22, 2020), lockdown 1 (March 23 to June 21, 2020), minimal restrictions (June 22 to Sept 20, 2020), lockdown 2 (Sept 21, 2020, to Jan 3, 2021), lockdown 3 (Jan 4 to March 21, 2021), and lockdown restrictions lifted (March 22 to March 31, 2021). In the analyses we examined changes in both attendance volume and consultation rate. We further compared changes in attendance trends to cancer-specific mortality trends. Finally, we conducted an interrupted time-series analysis with the lockdown on March 23, 2020, as the intervention point using an autoregressive integrated moving average model. FINDINGS: From 561 611 eligible individuals, 7 964 685 attendances were recorded. During the first lockdown, the median attendance volume decreased (-35·30% [IQR -36·10 to -34·25]) compared with the preceding pre-pandemic period, followed by a median change of 4·38% (2·66 to 5·15) during minimal restrictions. More drastic reductions in attendance volume were seen in the second (-48·71% [-49·54 to -48·26]) and third (-71·62% [-72·23 to -70·97]) lockdowns. These reductions were followed by a 4·48% (3·45 to 7·10) increase in attendance when lockdown restrictions were lifted. The median consultation rate change during the first lockdown was 31·32% (25·10 to 33·60), followed by a median change of -0·25% (-1·38 to 1·68) during minimal restrictions. The median consultation rate decreased in the second (-33·89% [-34·64 to -33·18]) and third (-4·98% [-5·71 to -4·00]) lockdowns, followed by a 416·16% increase (409·77 to 429·77) upon lifting of lockdown restrictions. Notably, across many weeks, a year-over-year decrease in weekly attendances corresponded with a year-over-year increase in cancer-specific mortality. Overall, the pandemic period revealed a statistically significant reduction in attendances for patients with cancer (lockdown 1 -24 070·19 attendances, p<0·0001; minimal restrictions -19 194·89 attendances, p<0·0001; lockdown 2 -31 311·28 attendances, p<0·0001; lockdown 3 -43 843·38 attendances, p<0·0001; and lockdown restrictions lifted -56 260·50 attendances, p<0·0001) compared with before the pandemic. INTERPRETATION: The UK's COVID-19 pandemic lockdown affected cancer health service access negatively. Many groups of individuals with cancer had declines in attendance volume and consultation rate across the phases of the pandemic. A decrease in attendances might lead to delays in cancer diagnoses, treatment, and follow-up, putting such groups of individuals at higher risk of negative health outcomes, such as cancer-specific mortality. We discuss the factors potentially responsible for explaining changes in service provision trends and provide insight to help inform clinical follow-up for groups of individuals at risk, alongside potential future policy changes in the care of such patients. FUNDING: Wellcome Trust, National Institute for Health Research University College London Hospitals Biomedical Research Centre, National Institute for Health Research Great Ormond Street Hospital Biomedical Research Centre, Academy of Medical Sciences, and the University College London Overseas Research Scholarship.