Concerns over Portugal's port quota have escalated into violence in Douro, after protestors broke the windows of the Instituto do Vinho do Porto last week.

The institute's president, Luciano Vilhena Pereira, has now been suspended ahead of his planned departure in October, The Drinks Business reports, leaving vice-president Paulo Meneses Osorio in charge in the interim.

Some 1,500 protestors were angered by the decision in July to cut the license per hectare permitted for port production from 110,000 pipes to 85,000.

This will create a shortfall of 2.7 million cases, marking a 30 per cent reduction in production compared with this year's port sales.

After they were driven back by police in the city of Regua, the protesters took handfuls of grapes and stomped on them in front of the building in a symbolic display of their frustration.

Paul Symington, managing director of the company that owns the Graham's, Warre's and Dow's brands of port, said many Douro farmers lost money in 2010, and this move will exacerbate the situation.

The producers' problems are also being compounded by sharp increases in the cost of the wine brandy that is used to fortify port, following the gradual withdrawal of the EU subsidy on distillation.

"This substantial cut in production will virtually eliminate any trade over-stock for 2012 and could eventually lead to some shortage of younger wines needed for ruby and reserve ports," he told the publication.

Mr Symington added that the Portuguese government has done very little to protect the 30,000 farmers in Douro, despite the fact that port is such an iconic export of the country.

News of the protests follows warnings earlier this year from Adrian Bridge of The Fladgate Partnership, which owns Fonseca, Taylor and Croft, that more producers are likely to consolidate within the next two years.