Varndell cut down Sam Vesty as he was waving to the crowd in premature celebration.
It means that when Newcastle meet Wasps in a do-or-die match on May 5 they must deny their opponents a bonus point and win by at least 24 points to send the Wycombe side into the Championship.
It was a bizarre ending to a remarkable second half turnaround as Bath ran in tries through Lee Mears, Stephen Donald and Nick Abendanon after trailing 9-0 at the break.
But Nicky Robinson landed his fourth penalty five minutes from time to earn the losing bonus point which could prove so crucial.
There was still time for Vesty's grandstanding and Varndell's crucial intervention, which needed a TV match official ruling.
It also cost Bath a bonus point, making their outside chance of a Heineken Cup spot even more improbable.
Bath had not scored a try in their last three Aviva Premiership matches, since beating Worcester 36-17 seven weeks previously.
Wasps needed three points from the game to banish the threat of relegation but still hanging over the club is the threat of administration - and a points penalty - if a new owner with at least £2million cannot be found to take over from Steve Hayes.
Bath made the early running but Wasps saw off the threat and Robinson's break at the other end nearly made a try for scrum-half Charlie Davies.
Robinson's next contribution was to send a perfectly judged kick rolling almost to the corner flag and when Mears' long throw to the tail failed to find its target, Wasps' attacking eventually provided the fly-half with a penalty kick to open the scoring after 14 minutes.
Bath went close at the other end when Donald barged through almost to the try-line and when the ball was swung left, Davies prevented Stuart Hooper from grounding near the flag.
But Wasps disrupted the resulting five metre scrum and 19-year-old No 8 Billy Vunipola surged away, combining with Christian Wade to lift the threat.
Growing in confidence, the visitors were soon threatening again, prompted by Robinson's accurate, teasing kicks.
Referee Dave Pearson was coming down heavily on Bath at the breakdown and Robinson took full advantage of their indiscipline, landing further penalties on 28 and 34 minutes.
Nothing was going right for Bath. On the stroke of half-time, Michael Claassens surged away from a ruck 30 metres from the posts and full-back Jack Wallace hared across to take man and ball as the scrum-half tried desperately to get the touchdown. Again the TV match official ruled the ball had been held up.
Eight minutes into the second half Bath finally crossed the try line, Mears cutting through on Abendanon's pass after Hooper nicked a line-out and Donald's long pass set Banahan surging through the Wasps defence. Barkley was just wide with the conversion.
They struck again after 54 minutes, this time from their own line-out.
Donald hurtled on to Claassens' flat pass and found a gap outside flanker Jonathan Poff. Barkley added the conversion to put his side ahead for the first time in the game.
Kyle Eastmond, signed from St Helens six months ago, made a cameo appearance as a blood replacement before Bath brought on five off the bench, including Duncan Bell and Pieter Dixon, making their last appearances at the Rec.
Wasps' best chance came when Wade broke away but Biggs made a last ditch tap tackle.
It was suddenly Wasps who were shipping penalties and as the second half count rose to 8-1 against them, Dominic Waldouck was sent to the sin bin.
Barkley hit the post with the kick, but Wasps could not escape from their 22 and Matt Carraro's long pass left Abendanon an easy run-in on 72 minutes.
Robinson narrowed the margin to five points soon after and then Vesty committed the cardinal sin of celebrating prematurely, unaware that Varndell was bearing down on him.
The extraordinary sequence of events continued with the big screen first showing 'TRY' and then being changed.
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