GPs should be “cautious” about vouching that their patients are fit to participate in marathons, their legal advisors have said.

The Medical Protection Society said rising numbers were signing up for high impact events, such as marathons, triathlons and extreme events overseas - including “fat camps” in the desert - when some were simply not fit enough.

The MPS issued its advice after a rise in queries from doctors who said they felt under pressure to sign fitness forms, without enough knowledge of whether the patient really was in good enough shape to cope with the exertion.

A record 253,930 UK applicants registered for a ballot place in Sunday’s London Marathon - and 56 per cent were from those who had never run a marathon before.