LONDON (Reuters) - Sixteen of Royal Birkdale's 18 holes have been altered for the British Open in July, organisers the Royal & Ancient (R&A) said on Tuesday.

Most of the modifications are focused on tightening rather than lengthening the course which, at 7,173 yards, will be playing only 155 yards longer than when the Southport links last hosted the third major championship of the year in 1998.

Despite the modest increase in length the R&A told a news conference that Birkdale would provide a fitting challenge for the world's best golfers.

"Believe me, it's long enough," said chief executive Peter Dawson.

Twenty new bunkers, 16 fairway and four greenside, have been added since American Mark O'Meara triumphed 10 years ago while significant mounding has been carried out to seven greens which will demand more varied and imaginative recovery shots.

Changes to the line of play have also been introduced at several holes, the most eye-catching at the ninth and 17th.

The ninth fairway has been moved 25 yards to the left while the par-five 17th has a new green 25 yards further back and raised above the fairway to create a more challenging approach shot, particularly for those attempting to reach the putting surface in two.

In addition, the R&A announced the continuation of the no-mobile phones policy for fans that was introduced when Ireland's Padraig Harrington lifted the Claret Jug trophy at Carnoustie in Scotland last year.

The 2008 Open starts on July 17.

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Card of the course

Hole Yards Par

1 450 4

2 421 4

3 451 4

4 201 3

5 346 4

6 499 4

7 178 3

8 457 4

9 414 4

OUT 3,417 34

10 408 4

11 436 4

12 184 3

13 499 4

14 201 3

15 544 5

16 439 4

17 572 5

18 473 4

IN 3,756 36

TOTAL 7,173 70

(Editing by Rex Gowar)