To analyse the efficacy and safety of two standard methods of endoscopic mucosal resection (EMR) for large non-pedunculated colorectal polyps, the investigators will compare submucosal injection-assisted endoscopic mucosal resection (SEMR) versus underwater endoscopic mucosal resection, without submucosal injection (UEMR).

Underwater endoscopic mucosal resection (UEMR) emerges as a variant of traditional resection, in which a submucosal cushion formation is not necessary. This technique was described in 2012 by Kenneth Binmoeller based on the physical floating effect of colonic mucous membrane by filling the colonic lumen using water instead of air. This "floating effect" conditions the natural separation of the colonic wall layers, allowing the distension of the submucosal space without the need to inject substances into it. On the other hand, performing resection in liquid medium conditions could drive the benefit of heat dissipation caused by the current, which would reduces thermal damage caused to the tissues.

Inclusion Criteria:

• Consecutive patients aged >18 years, diagnosed with a large (>=20 mm) non-pedunculated colonic polyp, who consent to endoscopic mucosal resection of the polyp and willing to participate in this study.

Exclusion Criteria:

• Polyps showing signs of deep submucosal invasion
• Inflamatory bowel disease with signs of activity
• Known or suspected pregnancy
• Patients unable or unwilling to provide informed consent