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Osborn poked home the winner after Benony Andresson stumbled in a one-on-one, leaving Stevenage’s defence unable to clear.

The semi-final had been a cagey affair, seemingly heading for a stalemate as neither side stamped their authority on the match.

Stevenage created the better of the chances in the first half and Jordan Roberts saw a hooked volley tipped over the bar by Corey Addai.

From the subsequent corner, Carl Piergianni headed wide.

Stockport grew into the game after the hour mark but Adama Sidibeh twice put efforts wide, and an Oliver Norwood free-kick flicked off the wall on to the roof of the net.

Odin Bailey saw a free-kick saved by Filip Marschall, but Osborn’s late intervention secured the crucial win.

Stockport boss Dave Challinor said: "It's one of the ugliest goals the play-offs has ever seen, but valuable as it puts us ahead in the tie.

"The jeopardy [in these games] is enormous. Stevenage are very good at making the game look like they want it to, they ask you questions.

"We've got to be a bigger threat but we've got a goal which puts us in a decent position.

"The tie is certainly nowhere near over."

Stevenage boss Alex Revell was frustrated with the late goal but insisted Boro is still in the fight for play-off glory.

"Of course it's frustrating, we could have dealt with the situation a lot better," he said, referring to the goal conceded.

"It's half-time, no matter what we would have had to go up to Stockport and try and win the game, it doesn't really change, even though they got the goal.

"The players want to win, they definitely don't want to concede a goal like that, it's up to us to pick them up and get them ready and prepared for Wednesday.

"The first half was really positive and we had a couple of moments in the second half when we could have got a shot away. Chances were few and far between today."