To date, there is a paucity of information regarding the effect of COVID-19 or lockdown on mental disorders. We aimed to quantify the medium-term impact of lockdown on referrals to secondary care mental health clinical services. We conducted a controlled interrupted time series study using data from Cambridgeshire and Peterborough NHS Foundation Trust (CPFT), UK (catchment population ~0.86 million). The UK lockdown resulted in an instantaneous drop in mental health referrals but then a longer-term acceleration in the referral rate (by 1.21 referrals per day per day, 95% confidence interval [CI] 0.41-2.02). This acceleration was primarily for urgent or emergency referrals (acceleration 0.96, CI 0.39-1.54), including referrals to liaison psychiatry (0.68, CI 0.35-1.02) and mental health crisis teams (0.61, CI 0.20-1.02). The acceleration was significant for females (0.56, CI 0.04-1.08), males (0.64, CI 0.05-1.22), working-age adults (0.93, CI 0.42-1.43), people of White ethnicity (0.98, CI 0.32-1.65), those living alone (1.26, CI 0.52-2.00), and those who had pre-existing depression (0.78, CI 0.19-1.38), severe mental illness (0.67, CI 0.19-1.15), hypertension/cardiovascular/cerebrovascular disease (0.56, CI 0.24-0.89), personality disorders (0.32, CI 0.12-0.51), asthma/chronic obstructive pulmonary disease (0.28, CI 0.08-0.49), dyslipidemia (0.26, CI 0.04-0.47), anxiety (0.21, CI 0.08-0.34), substance misuse (0.21, CI 0.08-0.34), or reactions to severe stress (0.17, CI 0.01-0.32). No significant post-lockdown acceleration was observed for children/adolescents, older adults, people of ethnic minorities, married/cohabiting people, and those who had previous/pre-existing dementia, diabetes, cancer, eating disorder, a history of self-harm, or intellectual disability. This evidence may help service planning and policy-making, including preparation for any future lockdown in response to outbreaks.