Three men are planning to row their way into the record books by travelling the full length of the River Thames in less than 39 hours.
Olympic gold medallist Tim Foster is backing the team
Andrew McCartney, 32, Martin Hirst, 33, and Alex Aldred, 36, all from Bedford Rowing Club, began rowing at 1000 BST on Friday, in Lechlade, Gloucs.
They plan to complete the 187-mile, non-stop challenge to Southend, in Essex, by 1200 BST on Sunday.
The rowers hope to raise £20,000 for Multiple Sclerosis and a Luton hospice.
A tracking device has been fitted to the rowing boat to be used as part of the verification of their attempt to be put forward to the Guinness Book of Records.
The device also enables their progress to be tracked via the Thames Challenge website.
The men have adopted a rota whereby two row while the third is the cox.
They are rowing a high-sided, coxed double scull which the men say will enable them to navigate the narrow and twisting stretch of the upper Thames as well as cope with the choppy conditions of the tidal estuary.
Team member Andrew McCartney told BBC London the trio had been training hard for up to 90 minutes every day to prepare.
Before the race he said: "We're confident that we're ready for the challenge and believe, weather and conditions permitting, that we're in with a chance of breaking the current record.
"It'll be tiring but the charities that we're doing this for are extremely worthwhile causes."
Money raised will go to the Multiple Sclerosis Society and Keech Cottage, a Luton-based hospice caring for terminally ill children and their families.
Olympic gold medallist Tim Foster is backing the attempt.