Background The extent of lymph node dissection in colonic cancer surgery remains arguable, and evidence from RCTs regarding extended lymph node dissection outcomes is lacking. This study aimed to compare the long‐term results of D3 lymph node dissection with those of D2 dissection. Methods This is a multicentre RCT. The aim is to enrol 768 patients with primary colonic cancer assigned randomly to D2 or D3 lymph node dissection. The trial is assessing the superiority of 5‐year overall survival as the primary endpoint in patients undergoing D3 lymph node dissection versus D2 dissection. Secondary endpoints include disease‐free survival, short‐term outcomes (30‐day morbidity and mortality), quality of complete mesocolic excision and lymph node dissection, pattern of lymph node metastasis and quality of life in patients following D2 and D3 lymph node dissection. Experience of 20 D3 and 20 D2 lymph node dissections is required for surgeons to participate in the trial. For surgical accreditation four non‐edited videos of procedures will be assessed. Patients will be followed up for 5 years after last patient enrolment. Intention‐to‐treat analysis will be performed. Discussion The results of this study will demonstrate whether extended lymph node dissection is superior to standard dissection in terms of oncological outcomes, and will also assess the impact of more extensive surgery on short‐term outcomes and quality of life. Protocol for a randomized clinical trial evaluating long‐term outcomes of D3 versus D2 lymph node dissection for colonic cancer. A trial to evaluate the extent of surgery