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The match will be shown exclusively live on BT Sport 1 HD and Ultimate from 6pm, with kick off from the home of football at 6.30pm.
The Foxes saw off Manchester United in the quarter-finals, with former Man City man Kelechi Iheanacho scoring twice in a 3-1 win.
Brendan Rodgers’ men have also seen off Stoke City, Brentford and Brighton & Hove Albion to reach the semi-finals for the first time since 1982.
The Saints have reached this stage of the competition for the second time in four seasons, with their FA Cup a welcome distraction from Premier League struggles.
Ralph Hasenhuttl has masterminded wins over Shrewsbury Town, holders Arsenal, Wolverhampton Wanderers and south coast rivals Bournemouth to reach the final four.
These two sides last met in January when goals from England hopefuls James Maddison and Harvey Barnes were enough for Leicester to claim all three points at the King Power Stadium.
The Champions League chasers would love a repeat result as they chase a maiden FA Cup against the 1976 winners.
The Saints have won two of the previous four meetings however – the Premier League record 9-0 defeat in October 2019 being their other loss – and will be confident they can match the high flyers.
For the likes of James Ward-Prowse and Danny Ings it could also be a possibility to showcase to England boss Gareth Southgate what they can do in big games ahead of this summer’s European Championships.
Watch Leicester v Southampton’s FA Cup semi-final clash live on BT Sport 1 HD and Ultimate from 6pm on Sunday 18 April.