Leigh Halfpenny handed Wales coach Warren Gatland a slight scare on Friday after he missed his side's captain's run ahead of their Six Nations clash with England.
ESPN understands that Halfpenny was too unwell to train with the rest of the squad in west London and he could now be a doubt for the match at Twickenham.
The fullback will be monitored by the squad's medical team ahead of Saturday's match.
If he is unable to recover in time to take the field it would be a considerable blow to Wales' hopes of dealing England a first home Six Nations defeat since 2012.
Halfpenny played a pivotal role as Wales opened their championship campaign with a commanding 34-7 victory over Scotland last weekend, scoring two tries and adding a further 14 points via a perfect performance from the kicking tee.
Gareth Anscombe would be the most likely beneficiary were Halfpenny to miss Saturday's match, while Rhys Patchell is also an accomplished fullback.