Shares in the group fell 98.4p, or 18.1%, to 444.6p as like-for-like sales in the last 10 weeks fell 1.5%, although total sales rose 6%.
Restaurant Group, which owns Frankie & Benny's, Chiquito and Coast to Coast as well as pub restaurants and concessions, said difficult trading at the end of last year had continued into early 2016.
Chief executive Danny Breihaupt blamed softening demand and weaker overall consumer confidence.
"Whilst still early in the year, our assessment is that this more challenging environment and recent trading patterns are likely to persist," he said.
"Although total sales will continue to increase as our new restaurants open and make good returns, in the current environment consistent like-for-like sales increases are likely to be difficult to generate."
The share price fall came in spite of higher revenue, profits and like-for-like sales in 2015 and a decision to increase the group's full-year dividend by 13% to 17.4p.
Revenue in the 52 weeks to December 27 rose 7.9% to £685mln, pre-tax profit lifted 11.2% to £86.8mln and like-for-like sales increased 1.5%.
Breihaupt said: "In common with most consumer businesses, we will again have some challenges to face in 2016.
"However, I am confident that the underlying strengths of our business will enable us to successfully navigate our way through this."