LONDON (Reuters) - British online retail sales growth picked up in June, but will slow in the second half of the year compared with the first, according to IMRG, the industry body for online retailers, and consultants Capgemini.

IMRG/Capgemini forecast on Friday 17 July that online sales would rise 12 percent in the second half of this year on the same period of 2008, down from their previous estimate of 15 percent.

Online sales rose 14 percent year-on-year to 22.9 billion pounds in the first half, including a 12.3 percent increase in June, up from an 8.2 percent rise in May.

"The online sales results for the first half of 2009 show a slight slowing of growth, as the recession weakens consumer spending on the Web," Mike Petevinos, head of consulting for retail for Capgemini UK said in a statement.

"(However) this should not mask the fact that online continues to be the driver of growth in the retail sector," IMRG/Capgemini said, noting that high street shopping sales saw only one month of year-on-year growth in the first half.

(Reporting by Mark Potter; Editing by David Cowell)