BackgroundEvidence regarding the rate of relapse in people with bipolar disorder (BD), particularly from the UK, is lacking. This study aimed to evaluate the rate and associations of clinician-defined relapse over 5 years in a large sample of BD patients receiving routine care from a UK mental health service.MethodWe utilised de-identified electronic health records to sample people with BD at baseline. Relapse was defined as either hospitalisation, or a referral to acute mental health crisis services, between June 2014 and June 2019. We calculated the 5-year rate of relapse and examined the sociodemographic and clinical factors that were independently associated with relapse status and the number of relapses, over the 5-year period.ResultsOf 2649 patients diagnosed with BD and receiving care from secondary mental health services, 25.5% (n = 676) experienced at least one relapse over 5 years. Of the 676 people who relapsed, 60.9% experienced one relapse, with the remainder experiencing multiple relapses. 7.2% of the baseline sample had died during the 5-year follow-up. Significant factors associated with experiencing any relapse, after adjustment for relevant covariates, were history of self-harm/suicidality (OR 2.17,CI1.15–4.10,p= 0.02), comorbidity (OR 2.59,CI1.35–4.97,p= 0.004) and psychotic symptoms (OR 3.66,CI1.89–7.08,p< 0.001). Factors associated with the number of relapses over 5 years, after adjustment for covariates, were self-harm/suicidality (β= 0.69,CI0.21–1.17,p= 0.005), history of trauma (β= 0.51,CI= 0.07–0.95,p= 0.03), psychotic symptoms (β= 1.05,CI0.55–1.56,p< 0.001), comorbidity (β= 0.52,CI0.07–1.03,p= 0.047) and ethnicity (β= − 0.44,CI− 0.87 to − 0.003,p= 0.048).ConclusionsAround 1 in 4 people with BD in a large sample of people with BD receiving secondary mental health services in the UK relapsed over a 5-year period. Interventions targeting the impacts of trauma, suicidality, presence of psychotic symptoms and comorbidity could help to prevent relapse in people with BD and should be considered in relapse prevention plans.Supplementary InformationThe online version contains supplementary material available at 10.1186/s40345-023-00302-x.