Background: The prostate-specific antigen (PSA) test can lead to under- and over-diagnosis of prostate cancer and has not been recommended for population screening. A fast MRI scan might overcome the limitations of PSA. IP1-PROSTAGRAM is the first study to evaluate the performance of a 15-minute non-contrast MRI for prostate cancer screening in comparison to PSA. Methods: IP1-PROSTAGRAM was a prospective, population-based, screen-positive paired-cohort study. Men aged 50-69 years in the UK were invited for prostate cancer screening through seven primary care practices or community-based recruitment. Participants underwent a PSA and MRI scan (T2-weighted and diffusion). MRI was scored using PIRADS version 2.0 without knowledge of PSA; screen-positive MRI (defined as either PIRADS score 3-5 or 4-5) were compared against a screen-positive PSA defined as ≥3ng/ml. If any test was screen-positive, a systematic 12-core biopsy was performed with MRI-ultrasound image-fusion targeted biopsy to MRI suspicious areas, as appropriate. Clinically-significant cancer was defined as any Gleason score ≥3+4. The primary outcome was the proportion of screen-positive MRI at different scores; important secondary outcomes were the number of clinically-significant and insignificant cancers detected. Results: 2034 men were invited to participate of whom 408 consented and 406 were screened by both PSA and MRI (10/Oct/2018-15/May/2019). The proportion with a screen-positive MRI (score 3-5) was higher than the proportion with a screen-positive PSA (17.7% [95%CI 14.3-21.8] vs. 9.9% [95%CI 7.3-13.2]; p < 0.001). A screen-positive MRI (score 4-5) was similar to a screen-positive PSA (10.6% [95%CI 7.9-14.0] vs. 9.9% [95%CI 7.3-13.2], p = 0.71). An MRI score 3-5 or 4-5 used to denote a screen-positive MRI, compared to PSA alone, detected 14, 11 and 7 clinically-significant cancers, respectively. There were 7, 5 and 6 clinically-insignificant cancers detected, respectively. No serious adverse events occurred. Conclusions: When screening the general population for prostate cancer, MRI using a score of 4-5 to define a screen-positive test, compared to PSA alone at ≥3ng/ml, could lead to more men diagnosed with clinically-significant cancer without increasing the number of men biopsied or diagnosed with clinically-insignificant cancer. Clinical trial information: NCT03702439.