Defender seals U's contract.Thompson will eligible for the U's game at Doncaster on January 5th, subject to the paperwork being ratified by the Football League and the FA.
"He has shown he is a player with tremendous ability and he has the potential to improve even further. He's a very good signing for us."
"Like Jack, Josh is a hungry young player who wants to better himself and I am delighted to get him in on a more permanent basis," he told the official website.
And Dunne was pleased to have secured the signature, following on from Jack Compton's arrival from the South Coast club.
His loan spell from Portsmouth ends at the turn of the year but he has turned that into a permanent deal after talks with manager Joe Dunne.
The twenty-one year old has agreed to an eighteen month contract at the Weston Homes Community Stadium, taking him up to the end of the 2013/14 season.
Josh Thompson will become a U's player on a permanent basis in the New Year.