A radical overhaul of the PRO12 could be required if Celtic sides are to challenge in Europe once again, says Wales great Shane Williams.

The European Champions and Challenge Cup semifinals get underway this weekend with the PRO12 fielding just one side in the last four of either competition -- the Newport Gwent Dragons take on Montpellier on Saturday in the continent's junior competition.

Toulon won back-to-back titles from 2013 to last term, but Leinster were the last PRO12 side to triumph in Europe's premier tournament in 2012.

"If you look at the teams and the calibre of the players in the PRO12, they should be competing," the former Ospreys wing told ESPN. "Some of the teams are very heavy in international players and you'd expect them to do better.

"Perhaps the PRO12 has not moved on in terms of the standard of teams over the last few years, while the Aviva Premiership and French league have gone up a step again. The Premiership looks stronger now than it's ever been. To have three teams in the Champions Cup semifinals just goes to prove that.

Shane Williams' London Marathon Q&A On Sunday Shane Williams will be running the London Marathon in aid of the Velindre Cancer Centre. How are you feeling ahead of the big day? I did it last year with very little preparation, but since then I've had the fitness bug. I do a lot of running, biking, and did the Wales Ironman last September. I'd like to think I'm in a better position this year. Anything you learned from last year? There were times during the marathon when I felt very drained and probably wasn't taking on sufficient drink and food. That was difficult, so that's something I've had to learn. Psychological/physical battle of running? What I find really difficult is running slow. Whenever I'd go for a run, I had a massive urge just to put my foot down and pick up the miles per hour. Slowing down my pace and controlling myself I found very hard, but it's all really tough. Plans for the day after? Relax! I'll stay in bed as long as I can. I remember last year my wife had to peel me out of bed. I was absolutely shattered.

"People want to see the stars play and that generates support, numbers and revenue. The only way they're going to be able to do that is to restructure [the PRO12] and perhaps not play [it] during the likes of the Six Nations and autumn internationals.

"That would mean you do have your full strength teams and hopefully that would generate a more competitive league, and really prepare these regional teams when they do play in Europe."

The qualification structure for the reformed Champions Cup -- which came into force in 2014 -- no longer guarantees PRO12 clubs spots at Europe's top table. And with Welsh regions struggling in domestic competition, big names are likely to be absent from next year's competition.

"It's disappointing that next year, the likes of the Blues, Ospreys, and Dragons likely won't be in the Champions Cup," Williams said. "It definitely needs a bit of change. Whether or not that's taking a few teams out of the league and making it smaller and more competitive, I'm not sure."

Saracens face Wasps in Saturday's first Champions Cup semifinal, before Leicester take on Top 14 giants Racing 92 a day later. Williams, however, is unlikely to be watching live -- the 39-year-old is taking on the London Marathon in aid of the Velindre Cancer Centre on Sunday.

"Every single person you speak to has been affected by cancer in some shape or form, so it wasn't very difficult to want to be involved with Velindre," Williams said. "It's all about fundraising. I don't set myself goals, just do what I can to raise as many funds and awareness as possible.

"If I can get under 3-and-a-half hours, I'd be very happy, but I really enjoyed the fact that I'd be running down the road and there'd be a 70-year-old woman running past me, or going over the finish line with a massive strawberry."