London - England limited overs coach Ashley Giles has denied his players tamper with the ball following controversial allegations by former captain Bob Willis.Willis told Sky Sports on Friday that Pakistan umpire Aleem Dar and New Zealand's Billy Bowden had ordered the ball to be changed while Sri Lanka were batting during their seven-wicket Champions Trophy win over England at The Oval on Thursday because it had been tampered with."Let's not beat about the bush --- Aleem Dar is on England's case. He knows that one individual is scratching the ball for England - who I am not going to name -- and that's why the ball was changed," Willis said.But ex-England spinner Giles, speaking Saturday ahead of his side's key Champions Trophy match against New Zealand in Cardiff on Sunday, said: "We don't tamper with the ball."With the situation the other day, the ball was changed because it had gone out of shape. We asked the question, the captain (Alastair Cook) asked that question to the umpires which he has a right to."The ball was changed, the rest is history."Giles added: "The most important thing is winning cricket matches and not what Bob Willis says."Reports in the British media have highlighted the role of Ravi Bopara in polishing the ball, a legitimate tactic, but Giles said the Essex all-rounder, who has starred with the bat in this tournament, had been selected for his cricket ability."There's even mention of one of our player's (Bopara's) specific role and that player is an extremely good cricketer, has had an extremely good series so far and we'd like to let him concentrate on playing his cricket."After England's tournament-opening win over defending champions Australia at Edgbaston, Australia stand-in captain George Bailey said he was "very surprised" by the speed with which Giles's side got the ball to reverse.However, Giles insisted Saturday an abrasive pitch at Edgbaston had played a key role in aiding England's use of reverse-swing against Australia.Prior to the Champions Trophy, New Zealand beat Giles's men 2-1 in a one-day series in England.New Zealand coach Mike Hesson said Saturday he'd seen nothing untoward during recent home and away matches against England."We played them in New Zealand and there were very abrasive surfaces and there was always going to be an element of reverse."Asked if England achieved reverse swing by legitimate means, Hesson replied: "I've got no idea how they achieve reverse swing and that's not my responsibility. The umpires are there to do a job and if they think something is done out of the ordinary then they will deal with it."From what I understand the other day they changed the ball because it was out of shape."On Thursday, Sri Lanka were 119 for two at the halfway stage of their reply to England's seemingly imposing 293 for seven when the ball was changed.The replacement ball moved little and Sri Lanka's Kumar Sangakkara went on to complete a match-winning hundred.Afterwards, Cook said: "The ball was changed because it was out of shape. The umpires make these decisions and you have to accept them. Sometimes you don't think they are the right decisions."Willis, an England captain in the early 1980s, said: "Have you ever heard about the batting side or the umpire complaining about the shape of the ball?"How naive does Alastair Cook think we are? He didn't want the ball changed. So why was it changed?"It is OK for the ball to scuff through natural wear and tear -- but against cricket's laws to use fingernails or other means to alter its condition," added 64-year-old former fast bowler Willis, one of only four England cricketers to have taken 300 Test wickets.Australian umpire Darrell Hair, together with West Indies' Billy Doctrove, docked Pakistan five runs for ball-tampering during a controversial Test against England at The Oval in 2006.Pakistan forfeited the match in protest but were subsequently exonerated by the International Cricket Council (ICC) and the fall-out cost Hair his Test career.However, officials in the England-Sri Lanka match didn't impose a penalty and the ICC explained, that as the umpires hadn't reported anything and no team had complained, they were not planning to take any action.