KABUL (Reuters) - A diplomatic row erupted on Wednesday between Afghanistan and key aid partners after Kabul declared "persona non grata" a Briton and an Irishman working for the EU and the U.N., accusing them of threatening state security by meeting Taliban insurgents.

With Afghan President Hamid Karzai away in neighboring Pakistan, a government official said acting European Union mission head Michael Semple and senior United Nations official Marvin Patterson had been expelled and must leave by Thursday.

"It is the government's last decision. They are persona non grata," the official told Reuters on condition of anonymity.

Western diplomats in Kabul closed ranks and insisted the row was merely a "misunderstanding," adding they hoped the pair would only have to leave for a short period.

Semple told Reuters that it would "not be appropriate" for him to comment on the matter at this time.

The government accused the pair, both old Afghan hands and experts in local languages and customs, of meeting Taliban members in the southern province of Helmand, heartland of Afghanistan's drug-producing poppy industry and an insurgency stronghold.

"Not only did they hold talks with the Taliban, but also had given them money," the Afghan official said. "It is not clear whether they were supporting the insurgency or not."

He said it was also unknown if the meeting was a personal initiative or if they were acting in an official capacity, but 50 Afghans -- some of them colleagues of the pair -- have been detained and investigated over their links to the matter.

NOT KNOWINGLY

Western diplomats said it was highly unlikely the pair would have knowingly met official Taliban representatives and it was more likely they were talking to tribal elders or chiefs.

Semple and Patterson have lived and worked in Afghanistan for more than a decade, even during the rule of the Taliban that was toppled by the U.S.-led invasion in 2001.

Both are considered highly experienced, hands-on experts on Afghanistan, valuable skills in a country in which scores of international and non-governmental aid organizations are attempting to run reconstruction and development projects.

Aid organizations and analysts say the biggest threat to humanitarian work in the country has been the growing Taliban insurgency. This is particularly so in the south and east, where remnants of Osama bin Laden's al-Qaeda network are stoking it.

The Afghan official said the meeting took place in Helmand's Musa Qala district, controversially abandoned earlier this year by British troops after a deal was struck with local elders to police themselves.

That agreement was criticized by the government after Taliban insurgents swiftly took control of the area until driven out two weeks ago when NATO and Afghan forces retook it.

Helmand is the heart of Afghanistan's drug-producing poppy industry and the EU and U.N. have a key role in the British-led eradication program.

Afghanistan's poppies produce over 90 percent of the world's heroin and the multi-million-dollar illegal industry it supports is said by analysts to be a primary reason for the Taliban's resurgence in the south and east.

The Afghan government, which has little support in the Taliban heartlands, insists publicly that it will not negotiate with the insurgents but frequent contacts are known to take place between unofficial emissaries.

Western governments also hold the line that the Taliban must not be negotiated with, but privately argue that dividing the insurgents and splitting the leadership is a legitimate strategy.

(With additional reporting by David Fox; editing by Roger Crabb)