Britain's FTSE extends gains into 4th day, ARM strong

* FTSE 100 up 0.5 percent

* Index up for 4th day, longest run since Oct

* ARM rallies as Apple (NasdaqGS: AAPL - news) secures China mobile (HKSE: 0941-OL.HK - news) deal

By Tricia Wright

LONDON, Dec 23 (Reuters) - Britain's shares rose for a fourth straight day on Monday as chip designer ARM was boosted by the prospect of more orders from key client Apple and optimism about U.S. growth underpinned the broader market.

Shares in ARM rallied 2.4 percent to 1,095.18 pence as Apple (EUREX: AAPF.EX - news) secured an agreement with China Mobile to sell iPhones through the world's biggest network of mobile phone users.

"Flows in ARM are largely biased towards the buy side, with accounts already long, increasing long positioning," Jordan Hiscott, senior trader at Gekko Global Markets, said.

A move above the record high of 1,111 pence set in May 2013 could pave the way for yet more gains, he said, with 1,250 pence his short-term target for the stock.

Sentiment surrounding ARM, whose chip designs power more than 95 percent of smartphones and most tablets, was also supported by a Christmas sales update from British department store group John Lewis, which said it sold one Apple iPad every 10 seconds at its stores during the past week.

The FTSE 100 was up 31.39 points, or 0.5 percent, at 6,637.97 points by 1151 GMT, on track for its fourth straight session of gains and its longest winning streak since October.

The UK benchmark rose 2.6 percent last week, snapping a six-week losing streak and notching its best weekly gain since July as strong U.S. GDP data cemented optimism the world's largest economy can stand on its own as monetary stimulus is withdrawn.

The FTSE 100 could yet enjoy a festive rally, a trend which has seen the FTSE 100 rise in all but two of the last 20 Decembers, according to Thomson Reuters Datastream. The index is currently down 0.2 percent this month.

"I think it could drift higher... I think people feel quite confident, and it signals (a fairly) good start to the New Year," said Edward Bland, director and head of research at Duncan Lawrie Private Bank.

The FTSE will trade for a half day on Tuesday and remain shut for the following two days for Christmas.

Volumes were very light going into the holiday, with the FTSE 100 having traded under a fifth of its 90-day daily average heading into mid-session.