Charity runners taking part in a 10 kilometre race at the weekend had already gone the extra mile by raising funds.

But by the end of the event, hundreds of them had gone an extra two miles, after a route marshal caused “mayhem” by disappearing from their post.

The first 300 of the 1,200 runners taking part in Britain's Bournemouth Bay Run went the wrong way as a result, running more than 3km further than they should have done and, in some cases, crossing the finishing line in tears.

The organisation of the race was so “shambolic” that at one point an elderly motorist drove onto the course, flashing her lights at oncoming competitors.

Bournemouth Borough Council, which organised the event on Sunday, said it was investigating why a marshal had not been at their post at a crucial point on the route.

The council said claims that the marshal had gone for a loo break were "incorrect", but refused to say exactly what had happened, other than that there had been “a confusion at the pre-race briefing over marshalling arrangements”.

The problem began when the front-runners approached the point at which they should have turned around and looped back on themselves at the halfway point of the race.

Instead of following a hairpin bend, they carried on running until some of the 300 realised they were in the wrong place and doubled back.

Hayley James, who is four months pregnant and from Poole, said: "To have a race of that scale with only one marshal on a point is inexcusable.

"We saw loads of people walking at the end, some were in tears, I felt so sorry for them - I felt like crying at the 10km mark."

Andy Isaac, from Bournemouth, said the event was "mayhem" with one point where an elderly woman managed to drive onto the route and was flashing her lights at oncoming runners.

Abi Harman, writing on Facebook, said: “Absolutely shocking. An event as big as this with little signposts and no marshall stood in the most important part of the course. What a joke. I have trained hard for this event and I was hoping to gain a PB [personal best] in doing so. I have let all the people who have sponsored me down. I have kept them up to date with my training and raised more money in doing so.

“I really feel for Karl Welch who came first in the 10K today, only to be told that there was no award ceremony because of discrepancies. He won fair and square and his achievement should be celebrated.”

The remaining 900 runners in the race followed the correct route after the marshalling problem was rectified.

Jon Weaver, head of resort marketing and events at the council, said: "Unfortunately there was some confusion with marshalling arrangements at one point, but it was a critical point. We apologise unreservedly to those front runners.

“We apologise unreservedly to all the runners who were affected and as a gesture of good will we have written to those who contacted us and were affected in yesterday’s 10k event and have offered the option of a refund on their entry fee.

“In 33 years of running the Bournemouth Bay Run this is the first time this has happened and as part of our debrief we will be analysing the arrangements carefully in this area to learn for 2016.

“We understand runners have trained for a long time for the event and it's hard for them and we do empathise with how they are feeling.”

Each of the runners had paid a £14 entry fee, which covers the cost of staging the event, with all of the £70,000 raised by competitors going to the British Heart Foundation.

Telegraph.co.uk