Mayor of Weymouth Caroline Nickinson said she would be at the start to see James off.
“Good luck Greg, we will give you a big send off as you set off from Weymouth to complete your 1,000km journey to Edinburgh,” she said.
Councillor Jon Andrews, from Dorset Council, added: “I know our community will be out in force to cheer him on.
“We’re proud to play a small part in supporting Comic Relief and the incredible difference it makes to the lives of the most vulnerable people.”
The breakfast show presenter will spend eight days in the saddle riding mostly solo with special guests and other Radio 1 presenters taking on the second seat during the journey.
His route for Radio 1’s Longest Ride will see him pedal over eight days through Wales, the Midlands and the north of England, to the finish on 20 March – Red Nose Day – in Edinburgh.