Lewsey, 30, suffered the injury to his right leg just before half-time in England's 14-9 semi-final victory over France on Saturday.
A scan last night confirmed Lewsey had suffered a grade one strain and he will miss England's clash with South Africa this coming weekend.
Lewsey hobbled off against France having given England an early lead with a try after just 80 seconds.
England coach Brian Ashton will name his team for the final on Wednesday and must now decide how best to replace the Wasps winger.
The likely choice is between recalling Mark Cueto or keeping Mathew Tait on the wing, where he finished Saturday's game, and starting with Dan Hipkiss at outside centre.
South Africa head coach Jake White said after the Springboks had beaten Argentina that England boasted a major advantage heading into the final given a number of their side had experience of winning the 2003 World Cup.
Lewsey was a key member of that team four years ago and Lawrence Dallaglio admitted today his absence on Saturday is a blow for the defending champions.
Dallaglio said: "He has played in one World Cup final and I'm sure he was looking forward to playing in another.
"He set the tone for the game at the weekend with a great opening try and put the pressure on France for the rest of the match.
"It will be a blow for him but one man's disappointment will be an opportunity for someone else.
"World Cups are not won by one person, they are won by a squad. I'm sure whoever comes in will do a fantastic job."
England confirmed Lewsey will be replaced in the squad ahead of the final.
A brief statement read: "Josh Lewsey sustained a grade one right hamstring strain at the end of the first half of Saturday's Rugby World Cup 2007 semi-final against France.
"He will take no further part in the tournament and will be replaced in the 30-man squad. The replacement is under consideration."