P&O say its cross-channel fleet was already working to capacity
Ferry companies operating out of Dover will be allowed to take 10% more passengers on their larger ships to help ease the current transport crisis.
The Maritime and Coastguard Agency said the measures applied to vessels in good weather and in daylight as they already carried enough lifesaving equipment.
But it said small boats could not carry people without proper certification.
Weekend plans by TV historian Dan Snow to mount a flotilla of small rescue craft were halted by the authorities.
The agency said: "In the light of the desire by operators of small craft to cross the Channel in order to pick up stranded tourists, small boat operators are being advised that such vessels must be properly certificated in order to make such a journey.
"Anyone operating such vessels should also be aware that such certification is only valid for UK waters.
"The operator will also need to seek clearance for their vessel to operate in French waters from the French maritime authorities."
A spokesman for P&O Ferries said the exemptions would not apply to them, as they were already operating at the highest permitted passenger level between Dover and Calais.
"We have five regular ships and we are also taking passengers on two freight ships.
"In fact, we still have some spare capacity," he said.