Blackwell has played 28 one-dayers for England

Warwickshire left-arm spinner Giles had the operation in December after coming home early from the Pakistan trip.

He is a major doubt for the first Test against India in Nagpur on 1 March.

"We are disappointed to lose a player of Ashley's calibre but remain hopeful he will play some part in the tour," chief selector David Graveney said.

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"Ian Blackwell is a talented player who performed well in the one-day series in Pakistan and this is a terrific opportunity for him to show that he can fulfil his potential against top-class opposition."

Despite a moderate recent return of 28 wickets in 14 Tests, Giles has become an integral part of England's bowling attack during their resurgence in the past two years.

He will stay behind to continue his rehabilitation at Edgbaston and there must be serious doubts over his participation in the nine-week tour, which also takes in seven one-dayers.

Giles has become a key member of England's bowling attack

England team doctor Peter Gregory said: "Ashley has been working extremely hard on his recovery programme and is making steady progress but he has not yet reached the stage where he is able to resume bowling again.

"The advice we received from Ashley's specialist is that it will take time to derive full benefit from the surgery he underwent in December.

"The operation was more extensive than we originally anticipated which is why we are taking his rehabilitation steadily.

"At this stage, we cannot put a timeframe on when Ashley might be able to join the England team."

If Giles does miss the three Tests, England will have to rely on 36-year-old off-spinner Shaun Udal - who claimed only three wickets at a cost of 92.33 apiece in Pakistan - and two Test novices in Blackwell, 27, and 23-year-old Monty Panesar.

Blackwell has at least played 28 one-day internationals, but has a poor record of 19 wickets at 34.05.

He is seen as a better batsman than his spin rivals, and has played some explosive innings for his county - whom he will captain next season.

But his 344 runs at 15.63 in one-day internationals do not bear that out.