Since late August, British commandos in the deserts of far southeastern Iraq have been testing one of the most serious charges leveled by the United States against Iran: that Iran is secretly supplying weapons, parts, funding and training for attacks on U.S.-led forces in Iraq.

A few hundred British troops living out of nothing more than their cut-down Land Rovers and light armored vehicles have taken to the desert in the start of what British officers said would be months of patrols aimed at finding the illicit weapons trafficking from Iran, or any sign of it.

There's just one thing.

"I suspect there's nothing out there," the commander, Lt. Col. David Labouchere, said last month, speaking at an overnight camp near the border. "And I intend to prove it."

Other senior British military leaders spoke as explicitly in interviews over the previous two months. Britain, whose forces have had responsibility for security in southeastern Iraq since the war began, has found nothing to support the Americans' contention that Iran is providing weapons and training in Iraq, several senior military officials said.

"I have not myself seen any evidence -- and I don't think any evidence exists -- of government-supported or instigated" armed support on Iran's part in Iraq, British Defense Secretary Des Browne said in an interview in Baghdad in late August.

"It's a question of intelligence versus evidence," Labouchere's commander, Brig. James Everard of Britain's 20th Armored Brigade, said last month at his base in the southern region's capital, Basra. "One hears word of mouth, but one has to see it with one's own eyes. These are serious consequences, aren't they?"

They are. Allegations that Iran or its agents are providing military support for Iraqi Shiite Muslim militias and other armed groups is one of the most contentious issues raising tensions between Washington and Tehran. Most gravely, U.S. generals and diplomats accuse Iran of providing infrared triggers for special explosives that are capable of piercing heavy armor.

Evidence of Iranian armed intervention in Iraq is "irrefutable," one U.S. commander in Iraq, Brig. Gen. Michael Barbero, told Pentagon reporters in August. The lead U.S. military spokesman in Iraq renews the allegation almost weekly in Baghdad.

Iraq's remote Maysan province is "a funnel for Iranian munitions," said Wayne White, who led the State Department's Iraq intelligence team during the war and now is an adjunct scholar at the Washington-based Middle East Institute. White said that in the first year of the occupation a well-placed friend had seen "considerable physical evidence of it, and just about everyone in al-Amarah knew about it." Al-Amarah is the commonly used name of Maysan province.

Here in Maysan, Jasim Alawa Salum, an Iraqi father of 10 whose home is in a warren of thatched farmhouses near the border, agreed. "All troubles come from Iran," he said, bending his head to show a wound from the 1980s Iran-Iraq war.

But Maj. Dominic Roberts of the Queen's Dragoons said: "We have found no credible evidence to suggest there is weapons smuggling across the border."

Asked why he could declare himself so confident that no arms were coming through, Labouchere mildly cited his confidence in Iraq's border force.

Guards at one of the 27 border forts now used to guard Maysan were dismissive of talk of military support from Iran. "It's just fabrication," insisted one, Haidar Hassan.

At one crossroads checkpoint, two border guards grinned awkwardly when a British desert patrol stopped in. No smugglers had come by, no untoward travelers, no problems, the guards said. The guards, however, come from tribes with a history of smuggling, and since the fall of Saddam Hussein, Iraqi border workers have redoubled their reputation for taking bribes.

To determine the truth of the charges, British commanders say, the British troops did something no other large-scale conventional unit in the U.S.-led coalition here has tried. They gave up their base.

Almost every night for months, rockets and mortar rounds had pounded Abu Naji, the outpost where British forces made their home outside Amarah, Maysan's provincial capital. In the base's last five months of use, 281 rockets or mortar rounds hit Abu Naji, Labouchere said.

Young soldiers would slip out of base at night to try to find the attackers. They would return in the morning as frustrated as when they left, he said. "The boys felt they were powerless," Labouchere said.

So the British forces packed up. The night before they left, mortars gave Abu Naji a farewell pounding.

About 5,000 townspeople gathered at the gates of Abu Naji on Aug. 24. When British troops pulled out that afternoon, the mobs moved in. Iraqi forces briefly tried to hold back the crowds, then gave way, said Maj. Charlie Burbridge, a British military spokesman at Basra. The mobs looted the base down to the bricks.

"This is the first Iraqi city that has kicked out the occupier!" loudspeakers at the local offices of Shiite cleric Moqtada al-Sadr trumpeted.

In their new mission, the British spread out over a desert carpeted with shrapnel, the legacy of the eight-year Iran-Iraq war that claimed the bulk of its 1 million dead here in the deserts of Maysan. Pressing all hands into duty, a former tank crewman became a medic; the regiment chaplain took the wheel as a fuel tanker driver.

If trouble in most of Iraq had inevitably followed foreign soldiers, the soldiers in Maysan didn't seem to hear anything coming. Attackers had lobbed a rocket or mortar round at them during their first week in the desert, but there had been nothing since, they said.

At the least, Labouchere said, "I am satisfied our presence will reduce" the dangers for the rest of Iraq.

Ultimately, however, the British can do little more than demonstrate that the borders are closed, Labouchere said. Save for that, he said, they find themselves trying "to prove a negative."