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Leinster boss Leo Cullen blames the PRO12 clubs' failure in Europe last year on the World Cup rather than the standard of the league championship.

Not one of the big guns in the PRO12 made the knock-out stages last year and there were fears that the financial muscle of England's Premiership and the Top 14 in France would leave the Celtic clubs behind.

But this time around at least two are on course to make it - Leinster and Munster - and possibly three or four could get there, with Glasgow and Connacht still in real contention.

Cullen is clear on why there is a difference this season. "A lot of the PRO12 teams are going to struggle with the World Cup," he said.

"If you think about it, in the English Premiership there are 12 teams so the World Cup dividing a 30 man squad among those 12 teams, it's a lot less in terms of the effect it has on those clubs.

"So they're geared up during that period, all preparing for the season, and it's the same in France with their 14 teams.

"For us, the World Cup affects in a much greater way. The same in Scotland, so Glasgow are missing a similar amount of players.

"It was a tough start for them (last year), very similar to us, they lost their first couple of games and the knock-on effect that that has.

"Ospreys would have been missing a lot of players, they didn't qualify from their group and didn't make the top six in the PRO12 either, hence why they're not playing in the Champions Cup.

"So teams like that have come back very, very strong in the PRO12 and in Europe as well.

"Ospreys are 20 points from 20 in the Challenge Cup and obviously they're going well in the PRO12, Munster have been sort of rejuvenated this year as well.

"So that's probably a big factor, I think. The World Cup probably affects the PRO12 teams more because in terms of percentage of those teams, it's far greater.

"With that many guys away, it's very hard to prepare and plan when the players aren't there, isn't it, whereas it doesn't affect the other teams in England and France quite as much."

For Cullen, the impact that a World Cup has - Leinster, for example, had over a full team on Test duty for the last finals - is more detrimental to the provincial cause than a perceived dilution of quality in the PRO12.

"They (the English and French clubs) aren't missing anywhere near the same amount of players," he argued.

"Maybe Toulon were a team in France who missed players because they have players from a lot of different nationalities, and they were affected.

"But for the other teams it doesn't seem to be quite as much of an issue. "But PRO12 teams have regrouped. Everyone was disappointed with, collectively, the showing from the PRO12 - and certainly it focuses the mind."