Director General of the World Health Organization (WHO) Margaret Chan said on Wednesday that the vaccine against A/H1N1 is still months away.
Chan's remarks cast doubt on the British government's claim that first stocks of A/H1N1 vaccine will arrive in Britain in August.
Chan told the Guardian that having a vaccine available is not the same as having a vaccine that has been proven safe.
Britain is to build a nationwide immunity to the disease, the biggest vaccination program of the past 50 years as the number of confirmed cases in Britain is moving closer toward 10,000.
The first doses of A/H1N1 vaccine are set to arrive in Britain next month, the British health authorities said on Sunday.
"Clinical trial data will not be available for another two to three months," she said.
Sixteen A/H1N1-related deaths and 9,718 confirmed cases of the disease have so far been registered in Britain, according to the latest figures from the Health Protection Agency.