TODD Blackadder could not conceal his anger as he criticised an "unforgivable" Bath performance after they crashed to defeat against west country rivals Gloucester.

Wales international Owen Williams converted Ed Slater's try five minutes into stoppage-time as Gloucester posted a 22-21 Aviva Premiership triumph and claimed a first away win of the season.

Semesa Rokoduguni looked to have done enough for Bath when he crossed four minutes from the end of normal time for his second touchdown at the Recreation Ground, but Gloucester were not to be denied.

"It was probably a nine out of 10 in terms of frustration," Bath rugby director Blackadder said. "It was sub-standard from us.

"We found ourselves in a position where we should have shut the game out, and we didn't. Credit to Gloucester, they deserved it, but from our side of it, that was just not good enough.

"We allowed them to build pressure, and we have just got to be better than that. It was error on error on error.

"The thing I look at is the energy, and when we go back and have a look at the video, we will be so frustrated. We missed the simple basics today, and it is just unforgivable."

Slater's try was awarded following repeated viewings from the television match official, before Williams' conversion sealed a first victory on the road for Gloucester since they beat La Rochelle in last season's European Challenge Cup semi-finals.

Fly-half Rhys Priestland added three penalties and a conversion for Bath after scrum-half and captain Willi Heinz scored two tries to build foundations for what ended up being only Gloucester's second win from their last 11 Recreation Ground visits in the Premiership.

Williams chipped in with a penalty and two conversions as Bath struggled to build on an impressive European Champions Cup success against the Scarlets nine days ago.

"It was one of those reality checks that you get," added Blackadder, who reported that England flanker Sam Underhill suffered a stinger injury down his neck, which was why he did not appear for the second half.

"I feel we are making a lot of progress, and then we put in a performance like that, which is the old Bath, where we don't fight, we don't turn up and we don't do the little things that give you the best opportunity to win games.

"I know we are better than that, and that performance is not good enough. We set high standards."

Gloucester, meanwhile, kept going until the end, with that perseverance rewarded as Slater and Williams combined to stun a 14,000 home crowd.

"I was on the ball, the ball was on the line. I'm pretty certain it was (a try)," Slater said.

"I've had some horror games here with Leicester. I know how difficult a place it can be.

"Our away form has been really poor, but today I thought we were the better side over 80 minutes. When it mattered, we held on to the ball and got the win.

"If we can have a bit more confidence in ourselves and be a bit more consistent, we can start to be a side that will challenge further up the table."

And Gloucester head coach Johan Ackermann added: "It was a nervy one. I watched Owen kicking in the warm-up, and he took one from that specific side and missed it - but he slotted the one that counted. He showed a lot of composure.

"I challenged the guys and said 'why can't we win away from home?'

"If you look at the games we lost at Harlequins, Sale and Leicester, we have played poorly. Today we stayed in the game, looked after the ball and went through a lot of phases."