* FY adjusted operating profit rises 39 pct to 184.1 mln stg
* FY Gross written premiums rise 21 pct, below estimates
* Says competitive environment continues to be intense
* Shares fall over 10 pct (Adds analyst, CFO comments, share movement, details)
March 1 (Reuters) - British insurer Hastings Group Holdings Plc reported a 39 percent jump in full-year adjusted operating profit as it provided insurance to more customers and increased its market share, although a slowdown in premiums written took some of the shine off results.
The company, which mainly sells motor insurance in the UK, said adjusted operating profit rose to 184.1 million pounds ($253.36 million) for 2017 from 132.1 million pounds a year earlier.
Gross written premiums rose 21 percent to 930.8 million pounds, about 2.5 percent below consensus estimates, missing both Barclays and RBC expectations.
Hastings' shares were down 7.1 percent at 290.4 pence at 0944 GMT, making them the biggest percentage losers on the FTSE Midcap Index.
"The results are good - but the stock is a growth stock, and growth has markedly slowed as the competitive environment has got tougher," Barclays analyst Alan Devlin told Reuters.
Hastings offers private car insurance, home insurance, motorbike and van insurance and premium financing and ancillary products. It has made headway in a competitive sector by focusing on selling motor insurance via price comparison websites.
Companies such as Admiral, Direct Line and esure also provide motor insurance in Britain.
"The competitive environment continues to be intense, with slower premium inflation since the end of the third quarter than that experienced in the first half of 2017 following the proposed Ogden rate review," said Gary Hoffman, the company's non-executive chairman designate, referring to the change in the way personal injury lump-sum payments are calculated.
Premiums rose in the first half of 2017, partly in response to the new rules for personal injury claims, but reversed course to fall in the next two quarters after Britain said it planned to change the way the payments are calculated, a move which would cut the size of those payments .
"As you look into the future, we would expect premium rate changes to follow claims inflation, with some uncertainty surrounding the proposed changes to the Ogden rate, and also from the planned whiplash reforms," Hoffman said.
Whiplash is a form of neck injury caused by a sudden jolt that snaps the head backwards, which insurers say has resulted in many fraudulent claims.
"It (whiplash reforms) is likely to be in 2019 now. The impact should be lower claims. The industry, as they see claims cost come down, will adjust premiums accordingly," CFO Richard Hoskins told Reuters.
($1 = 0.7266 pounds)
(Reporting by Noor Zainab Hussain in Bengaluru; editing by Amrutha Gayathri, Larry King)