Holbeach United suffered an agonising defeat in Monday’s penalty shoot-out.
After two hours without a goal it all came down to spot-kicks but the Tigers were beaten 6-5 in sudden death.
Lee Beeson had a penalty saved and Josh Ford hit the crossbar before Jake Clitheroe’s kick was also kept out, allowing Ollie Medwynter to win it.
Division One champions Peterborough Sports created history as the first team from outside the top flight to win the Knockout Cup.
Holbeach striker Ford had the first attempt of the game with a 20-yard drive which flew over the bar.
But then Peterborough Sports took control of the first half with Jordan Macleod taking centre stage.
His far-post header was comfortably held by Travis Portas. Then a couple of long-range free-kicks missed the target.
Macleod then cut inside from the left wing and fired towards the Tigers’ goal but Nick Jackson was in the right place at the right time to clear inside the six-yard box, just ahead of Portas.
Dan Lawlor sent another set-piece off target while Josh Sanders nodded wide and Dan Clements saw a shot held by Portas.
Holbeach had more chances in the second half as assistant manager Andrew Tidswell sent a volley over the bar to signal their intent.
Ford’s weak effort was held and then his cross created the first clear-cut chance but Clitheroe sent a header too high.
Macleod was denied again but the Tigers continued to push forward as Beeson’s free-kick was nodded wide by Ollie Pinner.
Lawlor’s drive deflected wide while Tidswell and Ford both went close towards the end.
In stoppage-time Holbeach very nearly won it as Beeson’s thunderbolt was superby tipped over by Luke McShane.
At the start of extra-time Sports substitute Avelino Vieira saw a shot blocked and a header was safely gathered by Portas.
Adam Jackson had an effort at the other end which went straight at McShane.
But the Sports keeper produced a magnificent save towards the end of the opening period. Tidswell’s volley was heading straight into the top corner of the net but McShane somehow tipped it away.
The only chance during the second period of extra-time saw Macleod have a shot pushed out by Portas.
4-4-2: Portas; Donald, Stevens, N Jackson, A Jackson; Beeson, Tidswell, Pinner (sub Cartwright 107 mins), Gale (sub Thomas 111 mins); Ford, Clitheroe. Subs not used: Dougill, Newcombe, Roberts.
4-5-1: McShane; Medwynter, Sanders, Gibbs, Bloodworth; Clements, Lawlor, Bucciero (sub Cobb 71 mins), Nyang, Macleod; Moreman (sub Vieira 90 mins). Subs not used: Sansby, Makate, Graham.
Tidswell (1-0); Sanders (1-1); Thomas (2-1); Nyang (saved, 2-1); Beeson (saved, 2-1); Lawlor (2-2); Ford (bar, 2-2); Clements (saved, 2-2); Donald (3-2); McLeod (3-3); Cartwright (4-3); Vieira (4-4); A Jackson (5-4); Bloodworth (5-5); Clitheroe (saved, 5-5); Medwynter (5-6).
N Jackson, Pinner (fouls).
Jamie Stevens – the skipper led by example at the heart of the defence to keep a clean sheet.
Wisbech Town (H) – Premier Division, Wednesday.