The two new infected farms are in the same area, near Dereham, as Whitford Lodge Farm, Hockering, where 35,000 chickens are being slaughtered after the disease was found there last week. A worker caught a mild form of the bird flu.
Their flocks, with 15,300 more birds, will be culled and a one-kilometre "restrictive zone" - limiting movements of poultry, eggs and poultry products - has been imposed around all three. Yesterday, even before last night's announcement, Japan banned poultry imports from Britain.
Dr Debby Reynolds, the Government's chief vet, said that yet more farms may be infected, adding that "we still cannot say that either of these two further farms are the index case" - the one where the infection started. The chickens at the two new farms, which have the same owner, are free range - heightening the chances that they will have caught the virus from wild birds. This will increase criticism of Dr Reynolds and ministers who repeatedly refused to order Britain's poultry indoors to minimise risk of infection.
The flu is a low-pathogenicity form of the H7N3 virus - not the virulent H5N1 - but experts say that it, too, could mutate to become deadly. The Government's defences - erected against H5N1 - should have caught this other strain if they had been effective.
Ministers and officials who boasted that Britain was "probably better prepared than any other nation" through "surveillance" and "early action" are facing the possibility of another foot-and-mouth style fiasco. Dr Reynolds admits the surveillance - testing wild birds for flu - failed to find any carrying the virus affecting the farms, despite the greatest bird flu monitoring exercise ever carried out in Britain.
The Independent on Sunday's investigation has established the testing itself may be to blame. Experts from Ohio State University and the University of Kalmar in Sweden say the Government's programme is turning up far too few cases of normal low pathogenicity flu, which is always in birds, to be credible.
The experts say samples must immediately be put in saline or preservative; the British tests fail to do this. The Department for Environment, Food and Rural Affairs says its tests are valid, but plans a trial to see if the methods used abroad are better.