Previous research suggests that sending non-participants a reminder letter, 1 year after their initial invitation, can improve coverage for bowel scope screening (BSS), also known as flexible sigmoidoscopy screening. We hypothesised that adding a general practitioner's (GPs) endorsement to the reminder letter could improve coverage even further. We conducted a randomised controlled trial in North West London, UK. Participants were screening-eligible men and women who had not responded to their initial BSS invitation at least 12 months prior to the trial period. Eligible adults were randomised in a 1:1 ratio to receive either a GP-endorsed reminder letter, or a standard reminder letter from June to August 2019. Logistic regression models were used to test the effect of the GP endorsement on attendance at BSS, adjusting for sex, clinical commissioning group, and local area socioeconomic deprivation. In total, 1200 participants were enrolled into the study and randomised to either the control (n = 600) or the intervention (n = 600) group. Those who received the GP-endorsed reminder letter were only slightly more likely to attend BSS than those who received the standard reminder letter (4% vs. 3%); this difference was not statistically significant (Adjusted OR = 1.30; 95% CI: 0.69, 2.43). Adding a GP-endorsement to the annual reminder letter did not have an effect on attendance at BSS. One possible explanation for this is that the endorsement used was not personalised enough. Future research should examine stronger GP-endorsements or other methods to promote uptake.