Background: Although surgical resection has been considered the only curative option for colorectal liver metastases (CLM), thermal ablation has recently been suggested as an alternative curative treatment. A prospective randomised trial is required to define the efficacy of resection vs ablation for the treatment of colorectal liver metastases. Methods: Design and setting: This is a multicentre, open, randomised controlled non-inferiority trial design with internal pilot and will be performed in tertiary liver centres in UK and The Netherlands. Participants: Eligible patients will be those with colorectal liver metastases at high surgical risk because of their age, co-morbidities or tumour burden and who would be suitable for liver resection or thermal ablation. Intervention: Thermal ablation as per local policy. Control: Surgical liver resection performed as per centre protocol. Co-interventions: Further chemotherapy will be offered to patients as per current practice. Outcomes Pilot study: Same as main study and in addition patients and clinicians' acceptability of the trial to assist in optimisation of recruitment. Primary outcome: Disease-free survival (DFS) at two years post randomisation. Secondary outcomes: Overall survival, timing and site of recurrence, additional therapy after treatment failure, quality of life, complications, length of hospital stay, costs, trial acceptability, DFS measured from end of intervention. Follow-up: 24 months from randomisation; five-year follow-up for overall survival. Sample size: 330 patients to demonstrate non-inferiority of thermal ablation. Discussion: This trial will determine the effectiveness and cost-effectiveness of thermal ablation vs surgical resection for high-risk people with colorectal liver metastases, and guide the optimal treatment for these patients.