Alex Gidman admits tonight's Pro40 showdown with arch-rivals Somerset is a "win or bust" affair for Gloucestershire.

Anything less than victory beneath the Bristol floodlights will almost certainly end the county's chances of lifting the Division One title in the competition's final year.

Beaten twice in four Pro40 outings already this season, Gloucestershire have ground to make up on leaders Sussex, who boast a 100 per cent record after three games.

And skipper Gidman conceded: "We can kiss goodbye to any chance we might have of winning the tournament if we don't beat Somerset.

"Any team that loses more than two games is going to struggle and we've already lost twice and had one game wiped out by the weather.

"There's no more room for manoeuvre and, as far as we are concerned, the competition has become a matter of sudden death."

Tonight's West Country spat is the final one-day game of the season at Nevil Road, after which the Gladiators meet Hampshire at the Rose Bowl (25 August), Yorkshire beneath the Headingley floodlights (September 10) and Nottinghamshire at Trent Bridge (September 27).

"We're going to have to win every single one of our remaining games if we're going to have a chance," said Gidman.

"It would be lovely to beat Somerset and keep our interest in the competition going, but we know there is no margin for error. It's win or bust."

Matches between Gloucestershire CCC and Somerset CCC are traditionally feisty affairs and Gidman expects regional rivalry to give tonight's encounter an additional edge.

"There's something very special about games between ourselves and Somerset," he said. "Beating them means a lot to our members and there is definitely an extra competitive edge whenever we come up against them.

"Honours were even when we met in the Twenty20 Cup earlier in the year. We beat them at their place, they beat us in Bristol and now we want to win the decider to get the upper hand."

Gloucestershire's one-day pedigree has invariably given them supremacy in meetings with Somerset in the 40 and 50-over competitions during the past decade, but Gidman believes the Sabres pose a more potent threat these days.

"There was a time not so long ago when they struggled in these games, but not anymore," he said. "Somerset are a much-improved side and they boast a good mix of experience and youth.

"They are one of the most improved one-day teams around and a team we respect enormously."

Gloucestershire have introduced a number of tactical changes to their Pro-40 line-up this season and, despite a mixed bag of results so far, Gidman insists these will stand the home side in good stead this evening.

Will Porterfield and Kadeer Ali are expected to open the batting and the captain said: "That left and right- handed partnership at the top of the order usually messes the opposition around and gets us off to a good start."

Of the decision to drop Hamish Marshall down the order and promote fellow New Zealander James Franklin to number four, he said: "We felt we needed to give James more time to build an innings rather than putting him under pressure to score immediately.

"He's like me in that he likes to get himself in, but can then be very destructive once he's seeing the ball.

"One of our biggest problems had been playing spin during the middle overs and moving Hamish to number five has helped in that respect."

Gloucestershire have named a 12-man squad, which includes Rob Woodman, but are expected to retain the side which lost by nine wickets to Sussex Sharks at Horsham on Sunday.