WASHINGTON, July 14 (Reuters) - Two major U.S. health insurance groups on Friday called on Senate Republican leader Mitch McConnell to drop from a healthcare bill a provision proposed by Senator Ted Cruz that allows insurers to offer stripped-down, low-cost healthcare plans.

"It is simply unworkable in any form and would undermine protections for those with pre-existing medical conditions, increase premiums and lead to widespread terminations of coverage for people currently enrolled in the individual market," America's Health Insurance Plans and the Blue Cross Blue Shield Association said in a letter to McConnell and Senate Democratic leader Chuck Schumer. (Reporting by Eric Beech)