EXPERTS have warned runners to take extra precautions ahead of tomorrow's Edinburgh Marathon as the Met Office predicts the May heatwave will continue over the weekend.

For the first time in its 10-year history, 50 marshals have been trained in cardio-pulmonary resuscitation by the British Heart Foundation, while the Scottish Ambulance Service will have 35 staff in attendance, in addition to 100 first-aiders, six doctors and 10 nurses on site.

After temperatures reached 27°C (80.6°F) in parts of Scotland this week, participants have been advised to stay safe in the sun by wearing sunscreen, drinking little and often and running in the shade wherever possible.

Marathon organisers will provide 131,085 bottles of water along the 26.2-mile route and 29,000 bottles at the finish line for the 12,635 entrants. The moves follow criticism from runners over a lack of water in 2009, when temperatures reached 22°C.

In 2010, when temperatures climbed to almost 25°C, Douglas MacFarlane, 52, from Edinburgh, died after taking part in the team relay event.

Temperatures in the capital are expected to reach 20°C or 21°C, but runners will be helped by low cloud and mist in the morning and easterly winds bringing cooler air.

Forecasters say it will be warmer in other parts of the country, with temperatures in inland Scotland and the west coast likely to hit 25°C or 26°C today and tomorrow.

A Met Office spokesman said: "Generally, it's a dry weekend, plenty of sunshine. The further east you are, the more likely you are to pick up a little bit of low cloud at times, particularly around eastern coastal areas."

Sun worshippers have flocked to beaches and parks during the unseasonably warm spell, with temperatures reaching a high of 27.3°C in Bishopton, near Glasgow on Thursday, well above the 13°C-15°C normally expected at this time of year.

Around 27,695 people are expected to participate in the two-day Edinburgh Marathon Festival this weekend, which includes a 10km, 5km, team relay and two junior races. Event organisers hope it will raise more than £4.5 million for charity.

The course is said to be the fastest in the UK due to the 40m drop in altitude from start to finish, but Glasgow-based physiotherapist Frank Gilroy, a six-time veteran of the event, advised participants to slow down in the heat.

He added: "If it's really hot, and you have been training in cold weather, then don't think you're going to do the same time."

Meanwhile, Prime Minister David Cameron is less impressed by the heatwave after he admitted on ITV's This Morning he had been struck down by hay fever as a result of the weather.

He said: "There's obviously one or two plants that, when they flower, hit me and so, just this morning, I woke up and I had it."

The good weather has also put coastguards and lifeguards on high alert as people try to cool off, with safety bodies advising members of the public to be safe in and around rivers, reservoirs and lochs. A teenager had to be rescued on Thursday when he got into difficulty in the River Nith near Nunholm Bridge in Dumfries. Police said he was not injured.