Sky Sports sources understand Watford defender Mike Williamson is set to undergo a medical at Portsmouth ahead of his proposed move to the Premier League.
It is understood the two clubs have agreed a fee in the region of £3million as Tuesday's 5pm transfer deadline draws closer.
Pompey manager Paul Hart is keen to bolster his backline having seen Sylvain Distin depart the club for Everton last week.
Williamson is poised to follow Tommy Smith in signing at Fratton Park from the Hornets this summer.
The 25-year-old stopper arrived at Watford from Wycombe in January and has since made 17 league appearances for the side.