All 8.5 million pupils in the UK would be immunised against swine flu at immunisation posts in every school, under plans being studied by ministers, the Guardian has learned.

In the biggest mass vaccination since the 1964 operation against smallpox, school nurses, health visitors and GPs would deliver the injections to five- to 16-year-olds at all 33,700 schools. The move would be part of a concerted government effort to minimise the harm caused by the expected second wave of the pandemic this autumn.

"The general principle of schools being the ideal, logical place to do this is well established. They have captive audiences," said one senior source involved in Whitehall planning.

While written parental approval would be necessary, high demand is expected, as long as the vaccine has been declared to be safe. However, the huge scale of the task has led to questions about whether there would be enough health professionals available to administer the jabs. For example, there are just 1,447 school nurses for the 25,000 schools in England.

The move comes as fresh doubts about the government's H1N1 preparedness timetable were raised yesterday when the World Health Organisation said that a clinically tested flu vaccine would not be ready this month, contrary to ministerial assurances.

Dr Marie-Paule Kieny, director of the WHO vaccine research initiative, said results from clinical tests on a new vaccine were not expected until September. Vaccines would then require regulatory clearance before being given to patients in Europe.

Her remarks cast serious doubt on a number of statements by the health secretary, Andy Burnham, and the chief medical officer, Sir Liam Donaldson, who have said the vaccine will be available this month. A Department of Health spokeswoman said: "The manufacturers have told us that we can expect the first supplies of the vaccine from Baxter in August and from GlaxoSmithKline later in September. This is not the DH's schedule – it is led by the manufacturers." But GSK admitted that it has yet to start clinical trials on people. "We are still at the discussion stage and are not putting a date on when clinical tests will start," a spokesman said.

The first wave of the pandemic appears to be on its way out. The number of new cases of swine flu in England and Scotland has fallen significantly, according to figures released by the Health Protection Agency yesterday. England recorded an estimated 30,000 new cases last week, down from 110,000 the week before, and in Scotland the estimated number of new cases fell from 1,500 to 1,050. Nine more people in England have died, taking the death toll to 36. Donaldson warned against complacency and said he was "pretty certain" of a second wave in the autumn.

The vulnerability of children to the H1N1 virus, the need to keep the pressure off family doctors' surgeries, and easy access to so many young people means schools are likely to be used. Vaccinating a primary school might take several hours but a secondary school several days.

Experts believe the second wave may start building up in England once schools return in September. But inoculation of pupils could not start until large supplies of the vaccine are available – which would be the start of October at the earliest, say health department sources. They also say that under-fives, another priority group, will be immunised at GPs' surgeries.

The Department of Health said last night that the exact form of the immunisation strategy has not yet been agreed. "We have not said that schools will deliver the vaccination programme for swine flu. Decisions have not been made on how the vaccine programme will be delivered, and the chief medical officer has said that he expects GPs to be the bedrock of the programme," the spokeswoman said.