London - England great James Anderson was forced to leave the field with a calf injury while bowling for Lancashire in their County Championship match against Durham on Tuesday.
With just a month until the Ashes start, any fitness concern surrounding the 36-year-old, the spearhead of England's Test attack, is sure to be a concern to the selectors ahead of a five-match series against arch-rivals Australia.
Anderson, who took his 950th first-class wicket on Monday, stretched and attempted to run in three times to shake off the injury but was eventually forced to leave the field on the third day of four at Sedbergh School.
An England and Wales Cricket Board statement said: "Jimmy Anderson has sustained tightness to his right calf. He will not bowl again in the game and will continue to be assessed."
Anderson, who is not involved in the ongoing World Cup as he no longer takes part in one-day internationals, is England's most successful Test bowler of all time.
His 575 wickets from 148 matches in the five-day game place him fourth in the all-time list of Test wicket-takers.
The first Ashes Test is scheduled to start at Edgbaston in Birmingham on August 1.
Australia hold the Ashes but they have not won a Test series in England since 2001.