Forster, who was at Celtic Park to watch Thursday's Europa League play-off with AIK, will compete for a place with Scott Bain and Craig Gordon.

The 31-year-old won three league titles and two Scottish Cups in a four-year spell under Neil Lennon.

"Fraser was available, he's knows the club, we know him," boss Lennon said.

"He had a wonderful time here the first time and he's still at his peak."

Forster spent two seasons on loan at Parkhead before joining on a permanent deal for £2m from Newcastle United in 2012. He made 197 appearances, as well as winning the first of his six England caps.

He has three years left on his Southampton contract but has played just once for the Premier League club since December 2017 and dropped to third choice behind Angus Gunn and Alex McCarthy.