The drug has been offered to those with close contact to swine flu
Experts have reported the first case of swine flu that is resistant to tamiflu - the main drug being used to fight the pandemic.
Roche Holding AG confirmed a patient with H1N1 influenza in Denmark showed resistance to the antiviral drug.
David Reddy, company executive, said it was not unexpected given that common seasonal flu could do the same.
The news comes as a nine-year-old girl has become the third to die in the UK with swine flu.
It is understood from her doctors at Birmingham Children's Hospital that she had underlying health conditions. It is not yet known whether swine flu contributed to her death.
Meanwhile, the Department of Health has announced a big jump in the number of patients in England confirmed with swine flu - up 1,604 since Friday, taking the UK total so far to 5,937.
Experts have been using tamiflu, also known as oseltamivir, in a bid to stop the H1N1 spreading in communities.
If taken early, it ensures that symptoms are mild and reduces the chance of a victim giving the illness to someone else.
This first reported case of resistance developed in a swine flu patient taking Tamiflu.
Watch and wait
Mr Reddy stressed that there were no signs of a tamiflu-resistant strain of H1N1 circulating in the community.
This is in contrast to seasonal H1N1 flu where a Tamiflu resistant strain emerged last year and is now widely circulating.
Experts fear if this were to happen, it could render tamiflu ineffective.
Another antiviral drug, called zanamivir or Relenza, made by GlaxoSmithKline, is also effective against swine flu.
The UK government has been stockpiling these antiviral drugs and currently has enough to treat half of the population, with a contract to bring that up to 80% as soon as possible.
Supplies of flu vaccine have also been ordered and the first doses could be administered in the autumn.
A spokeswoman for the Health Protection Agency said: "The Health Protection Agency continues to watch for antiviral resistance and will be carrying out regular sample testing throughout this outbreak.
"We have been monitoring antiviral drug resistance since the beginning of this outbreak. Routine sampling in the UK has shown that there is currently no resistance to oseltamivir or zanamivir."
Virologist Professor John Oxford said: "I'm not surprised about this finding. The question is whether it is going to spread. We will soon know the answer."