The 26-year-old is having a medical at St James’ Park after Newcastle have had a bid, believed to be around £1million, accepted for the centre-half, according to Sky Sports News.
Williamson has failed to make an impact at Portsmouth since joining the club in a £3million deal from Watford in September.
Newcastle boss Chris Hughton earlier told the Northern Echo: “There has been an offer made which has been accepted and at the moment we’re just working on the lad’s terms.
“He’s a lad with good pedigree.
“He did very, very well at Watford and I have known about him for a while. We were interested in him before he went to Portsmouth but his games have been limited there.”
