Town have taken up the one-year option on left-back Jonas Knudsen’s contract.
The Danish international’s deal, which was otherwise set to be up next summer, stipulated that the option which takes it to the end of June 2019 had to be activated by the end of this month.
"The club have taken up the option. Jonas has been excellent for us,” boss Mick McCarthy told IFollow.
"He has bought into everything TC [Terry Connor] and I have tried to do and has got better and better.
"He has got himself in the Denmark squad now and I really hope he makes it to the World Cup.”
Cole Skuse, Bartosz Bialkowski, Teddy Bishop, Adam McDonnell, Luke Hyam, Kieffer Moore and Kevin Bru are among those whose contracts are also due to come to an end this summer but with options for a further season which have to be taken up by the end of May.
Striker David McGoldrick’s terms are also up in the summer but with no option for an additional year.
Elsewhere, former Blues skipper Carlos Edwards, 39, has joined Thurlow Nunn Division One side Woodbridge Town.