Background: Trials have identified antimicrobial stewardship (AMS) strategies that effectively reduce antibiotic use in primary care. However, many are not commonly used in England. We co-developed an implementation intervention to improve use of three AMS strategies; enhanced communication strategies, delayed prescriptions, and point-of-care C-Reactive Protein testing (POC-CRPT). Aim: To investigate the use of the intervention in high-prescribing practices and its effect on antibiotic prescribing. Design and setting: Nine high-prescribing practices had access to the intervention for 12 months from November 2019. This was primarily delivered remotely via a website with practices required to identify an ‘Antibiotic Champion’. Methods: We compared routinely collected prescribing data between intervention and 45 matched control practices using a difference-in-differences analysis. Intervention use was assessed through monitoring. Surveys and interviews were conducted with professionals to capture experiences of using the intervention. Results: There was no evidence that the intervention affected prescribing. Engagement with intervention materials differed substantially between practices and depended on individual Champions’ preconceptions of strategies and opportunity to conduct implementation tasks. Champions in five practices initiated changes to encourage use of at least one AMS strategy, mostly POC-CRPT; one practice chose all three. POC-CRPT was used more when allocated to one person. Conclusions: Clinicians need detailed information on exactly how to adopt AMS strategies. Strategies may be used more when allocated to one or two individuals. Remote, one-sided provision of AMS strategies is unlikely to change prescribing; initial clinician engagement and understanding needs to be monitored to avoid misunderstanding and sub-optimal use.