The study of 29 European countries by scientists at the London School of Hygiene and Tropical Medicine said authorities must urgently tackle shortcomings in preparation for vaccine and antiviral drug distribution, insufficient stockpiles, and incoherent plans for border controls.
It warned that many European governments risked "chaotic service responses and public anxiety" by leaving it up to regional or local authorities to organise drug delivery during a pandemic and giving them little guidance.
"Our findings show that even in Europe, which may be better prepared than some regions, considerable gaps and inconsistencies persist and several areas of operational planning have not been addressed," the study published in the October issue of the WHO Bulletin said.
The authors concluded that "the remaining gaps and inconsistencies need urgent attention" despite outwardly strong government commitment in Europe and strengthened planning since their last evaluation in 2005.
Scientists fear that the H5N1 strain of bird flu, which emerged in humans in Asia in the 1990s, could mutate into a more virulent form that could easily be passed between humans, triggering a global flu pandemic with the potential to kill millions.
Some 329 people have contracted H5N1 bird flu since 2003, of whom 201 have died, according to the WHO. Most of the human cases have been in Asia but it has also spread to the Middle East and Nigeria.
The outbreaks triggered a WHO-led drive in recent years to bolster preparedness for a pandemic, including stockpiling of antiviral drug treatments, greater vaccine research and advance emergency planning.
The study said only half the countries had developed full storage and delivery strategies for antiviral drugs, particularly as a preventive treatment to slow the spread of pandemic flu.
"The issue of how to deliver antivirals within 48 hours to individual patients remains largely unresolved," it added.
Vaccine strategies had "important gaps" and only a minority of the countries surveyed had guidelines on vaccine storage, distribution and administration during an emergency.
One country did not have plans for pandemic vaccination at all.
There is currently no known vaccine against pandemic flu as one would have to be developed and produced rapidly to match the emerging strain at the time.
The authors also said that shortcomings in planned checks on travellers were likely to be "politically volatile during a pandemic."
Planning for border controls, quarantines or travel restrictions varied widely, and many countries ignored WHO recommendations favouring exit-screening for travellers to stop the spread of pandemic flu, the study found.
Only half of the countries explictly planned to coordinate their border controls with neighbours, 19 favoured screening travellers on entry, while 10 did not plan to screen travellers at all, it added.
The study was meant to cover the 27 European Union member states, Norway, Switzerland and Turkey, but one unidentified nation was not included in the final analysis.