Pompey have completed the signing of versatile midfielder Shaun Williams on a one-year deal.
The 34-year-old Republic of Ireland international officially becomes a Blues player when his Millwall contract expires at the end of the month.
Head coach Danny Cowley said: “It’s a really good signing for us and we’re excited to add him to our group.
“Shaun’s played in the Championship for a lot of his career and is someone with international experience.
“But he’s also appeared for League One clubs and has been promoted from this division, so he has plenty of game intelligence and know-how.
“He knows how to receive the ball and gives us more left-footed balance in the group, which is important.
“From a tactical point of view it’s a really nice fit. He loves to come out into a false full-back position, which then allows our left-back to get high and wide.
"Shaun’s played in the Championship for a lot of his career and is someone with international experience."
“He also brings a lot of experience to our changing room. We have to sign players who will thrive being at a big club with high expectations.
“One of the things I look for is how they respond to adversity. He’s never going to be afraid to get the ball and is incredibly brave.”
Williams – who is also comfortable in defence – began his career in his homeland with Drogheda and Sporting Fingal, enjoying cup glory and Europa League football with the latter.
He moved to MK Dons in March 2011 and impressive displays during the 2012/13 campaign saw him named Player of the Year by both supporters and team-mates.
Williams then signed for Millwall at the start of 2014 and appeared at Wembley when they beat Bradford to win the 2016/17 League One play-off final.
He struck 22 goals in 295 appearances for the Lions, featuring 22 times in the Championship last term.
Williams has been capped three times by Republic of Ireland and netted in a 2018 Nations League clash with Wales.