Gloucester reached their first English Cup final for 13 years after defeating bitter rivals Leicester 16-11 in a thrilling Powergen Cup semi-final at Franklin's Gardens.
Paramore and Delport celebrate after a famous victory
Second-half tries from Marcel Garvey and Thinus Delport proved to be enough for Gloucester, after two first-half penalties from Tim Stimpson had given Leicester a 6-3 half-time lead.
Josh Kronfeld's late try put Leicester within a point of victory, but Stimpson's missed conversion gave Mercier the chance to extend Gloucester's lead with a crucial penalty.
Leicester: (6) 11
Pens: Stimpson 2
Gloucester: (3) 16
Tries: Garvey, Delport
Pens: Mercier 2
Even then, Leicester almost pipped their opponents after a series of line-outs and, controversially, uncontested scrums deep in the Gloucester 22.
Leicester started well and went ahead through a Stimpson penalty on three minutes after Mercier's high tackle on Ollie Smith.
Gloucester increased the tempo, turning down several kickable penalties in favour of territory, and it took some borderline Leicester defence to keep them out.
Tigers flanker Josh Kronfeld was lucky to avoid a yellow card after killing the ball in front of the posts.
We had to work very hard, but our guys
knew what was at stake
Gloucester coach Dean Ryan
Mercier landed the resulting penalty to bring his side level.
During injury time, Stimpson went just wide with a 60-metre penalty attempt, but landed an easier attempt a minute later to put Leicester ahead.
In the early stages of the second-half Leicester's pack moved up a gear but Gloucester's defence held firm.
The Cherry-and-whites finally broke through on 55 minutes with a brilliant try from young winger Marcus Garvey.
Full-back Thinus Delport joined the line at pace, and his incredible pass behind his back gave Garvey the space to beat three defenders on his way to the line.
Mercier puts over the crucial penalty
Mercier missed the conversion, but Gloucester followed up with a second try just three minutes later.
Gloucester secured great ball off the top of the line-out and Delport outpaced the defence to put Gloucester firmly in command.
Mercier again missed the conversion, and Leicester - still only seven points behind - pressed hard in search of a try.
With six minutes of normal time remaining, flanker Kronfeld went over from a traditional Leicester driving maul.
But Stimpson was unable to add the conversion, and Mercier's penalty two minutes from time put Gloucester further in front.
We didn't play well on the day and we didn't deserve to win
Leicester boss Dean Richards
As the match went into injury time Leicester staged another fightback and camped in the Gloucester 22 after a series of scrums and penalties in their favour.
The Tigers launched wave after wave of rolling mauls and referee Steve Lander warned Gloucester that a penalty try was imminent.
Hooker Dorian West drove over the line but was held up by some desperate defence.
And after Gloucester prop Rodrigo Roncero went off, Lander ordered uncontested scrums - much to the disgust of a Tigers pack in the ascendancy.
After a final all-out attack, the ball was turned over and Gloucester secured their spot at Twickenham.
Leicester: Stimpson, Murphy, Smith, Kafer, Lloyd, Vesty,
J Hamilton, Rowntree, West, Garforth, M Johnson, Kay, Corry,
Replacements: Chuter, Skinner, W Johnson, Freshwater, Booth,
Gloucester: Delport, Garvey, Simpson-Daniel, Paul, Beim,
Mercier, Gomarsall, Roncero, Azam, Deacon, Fidler, Cornwell,
Boer, Hazell, Forrester
Replacements: Stuart-Smith, Amor, Catling, Fortey, Woodman,
Referee: Steve Lander (RFU)