MONTREAL—With record demand, exactly 27,000 runners are registered to run in the Montreal Marathon on Sunday. However, that clearly isn’t enough.

A black market has emerged where bibs and numbers for Sunday’s race are being sold illegally because the cap has kept out runners who are willing to pay to take part. Organizers are scrambling and they say they have no solution.

Starting on the Jacques-Cartier Bridge at 8:30 a.m, the 42-kilometre race winds around Ile Ste-Helene, through Old Montreal to Jarry Park and detours around the Big O before ending at Lafontaine Park.

Camille Usher loves to run and she says she loves the rush of a good marathon.

“I think there's something to a marathon that people love. It's a challenge, it's an accomplishment and when you finish it's a feeling that's unlike anything else,” said Usher, who works at the Running Room.

Representing 40 countries, demand was so high for the 2012 race that registrations were closed six weeks before Sunday’s event. For those who have since decided to drop out—due to illness, injury or lack of preparedness—many are selling their spots in the sold-out competition.

Organizers of the marathon are not amused.

Organizer Peter Englehart said that switching bibs is against the rules and regulations and can also be a safety problem.

"At the end of the day the most important factor is safety and when people start switching bibs and then identities get confused that's something that we're very sensitive to,” said Englehart. “Because in the end we're just trying to provide a safe fun experience for all the runners.”

What's of particular concern is if a runner needs medical attention during the race. With incorrect identification there could be a delay in notifying the next of kin during a medical emergency.

Despite the risks, Usher says she knows a lot of people who drop by the Sherbrooke St. shop who would be willing to buy a spot.

“There are a lot of people that want to buy bibs and they have to go through a bit of a black market to get them. It's just a little weird for people who just want to run,” said Usher.

A search of Craigslist showed more than two dozen listings of those selling and buying bibs—more are up for offer on Kijiji.

Some racers say it is also unfair, since people who are officially in their age group could turn out to be much younger, skewing race times.

This weekend organizers plan to ask for photo ID before handing over bibs, but are not sure that will be enough of a deterrent.