"Our first formulation had to be stored and shipped at minus 80 degrees. We have now, in the meantime, a formulation which is not yet approved... which can be stored at two to eight degrees," Sahin told a Financial Times conference panel, adding that data packages were being prepared for regulators.
The probe is likely to be independent and carried out on behalf of the Cabinet Office, the BBC said. Reuters has not been able to independently verify the reports.
In his first comments on the row on Sunday, Cameron said that in his representations to government he was breaking no codes of conduct and no government rules.
"The response to Covid and to the financial aspects [of the crisis] has been fragmented, and geopolitical divides are not helping," Guterres told the FT.