Portadown has the highest proportion of vacant shops in Northern Ireland, according to a new survey.The study suggests that one in five outlets in the town is lying empty.The research was carried out by commercial property company Lisney in July and August.Seventeen towns and cities across Northern Ireland were surveyed,with the lowest percentage of empty shops being in Craigavon.Portadown was top with 21% of commercial properties being empty.It was closely followed on the list by Bangor, Antrim and Belfast.At the other end of the scale Cookstown and Newry had much higher occupancy rates.The survey said about one in 10 shops are vacant in Newtownards, Omagh, Londonderry and Enniskillen.The survey results show that overall Northern Ireland\'s retail occupancy rates are lagging behind the rest of the UK.The average vacancy rate was 14.4% which compares to a UK average of 11.2%.Lisney said the differences from town to town reflect over-development in some areas.As an example, the company said Cookstown, which has a well established High Street and one small retail park, has a vacancy rate of just 6%.Meanwhile, Portadown\'s high vacancy rate is influenced by having three shopping centres in the area.The survey also found that rents for shops in Northern Ireland have dropped by between 30 and 50% since the peak of the market in late 2007.The fall in rents has meant that discount retailers can now afford to set up on prime sites which is leading to higher vacancies in so-called secondary pitches.Lisney predict that rents and vacancies in these secondary areas are likely to increase in the medium term.