UPDATE 3-UK retailer Game's sales dip worsens; shares fall

* 21 weeks to June 27 sales down 15 pct

* Says has seen good margin growth, maintains guidance

* Sees H1 underlying pretax profit of 13-16 mln stg

* Shares down 9 pct (Adds further detail, comments by CEO, analyst; updates shares)

By James Davey

LONDON, July 2 (Reuters) - British video games retailer Game Group Plc (GMG.L) on Thursday reported an accelerating decline in sales, only partly offset by better profit margins, sending its shares as much as 11 percent lower.

The group, which trades from 1,367 stores, concessions and franchises in nine European countries and Australia, said like-for-like sales fell 15.4 percent in the 21 weeks to June 27.

This compares with a fall of 6.3 percent for the 11 weeks to April 18, indicating underlying sales have slumped by about 25 percent over the latter 10 weeks.

Chief Executive Lisa Morgan blamed the decline on an exceptionally strong release schedule in the previous year. She said the software release schedule for the second half of the current year was much stronger than the first half, with titles including Wii Fit Plus, Singstar: Take That, and Forza Motorsport 3 due for release.