Jannik Vestergaard has 18 caps for Denmark

Southampton have agreed a £18m fee with Borussia Monchengladbach for defender Jannik Vestergaard, according to Sky sources.

The Denmark international, who went to the World Cup in Russia but did not feature, will undergo a medical this week at the Premier League side.

Vestergaard made 32 Bundesliga appearances for Monchengladbach last season as they finished ninth in the Bundesliga, and has spent his entire career in the German top flight following previous spells with Werder Bremen and Hoffenheim.

Personal terms are not expected to be a problem for the 25-year-old, who could become the Saints' third signing of the summer following the arrivals of Mohamed Elyounoussi and Stuart Armstrong.

The 26-year-old Armstrong, an attacking midfielder, signed from Scottish champions Celtic for a reported fee of £7m while Norway international forward Mohamed Elyounoussi arrived from Basel for £16m.

2:02 As Southampton sign Stuart Armstrong, we take a look at some of his most memorable strikes for Celtic As Southampton sign Stuart Armstrong, we take a look at some of his most memorable strikes for Celtic

In the other direction, Ajax signed Southampton midfielder Dusan Tadic on a four-year deal for £10m while Rangers are keen on bringing Steven Davis back to Ibrox.

Southampton narrowly avoided Premier League relegation last season, defeating Swansea on the penultimate day before securing their status against Manchester City.

They get their 2018-19 season underway against Burnley on August 12 at St Mary's.