Oil smuggling has been a key source of revenue to Iraqi President Saddam Hussein. Dozens of small ships attempt to run the UN blockade by hugging the coasts of Iraq and Iran, then selling their illegal cargoes in the Gulf States.

Watch Jon Leyne's report

The Royal Navy's HMS Cumberland is involved in enforcing the ban.

On the day I was on board the ship she had an early-morning surprise for the oil smugglers. The ship's helicopter had flown out to check on an ocean-going tug that had been seen trying to run the blockade.

Final planning in the operations room

Details of the morning's contact, including the name and type of the ship were reported to the command.

In the operations room, the ship's commander said the tug had been involved in what looked like classic a smuggling posture and on that basis he gave authorisation to board the tug. Months of routine watching and waiting had finally borne fruit.

Ordinary seamen - a stoker, a radio operator, and even a member of the catering team were involved in the operation which was deliberately designed for maximum dramatic effect.

Aboard the blockade buster

Naval officers dramatically board the tug

The tug's mainly Filipino crew were left to wait in silence as the boarding party inspected the boat from stem to stern, looking for anything to show how the illicit trade is orchestrated by the Iraqi Government.

The evidence found on board was overwhelming. The ship was so overladen with oil that the decks were almost under water.

An extra oil tank had been rigged up on the stern and oil was even been stored in the tug's fresh water tanks.

The smuggling route was partially erased

"We found a large amount of gasoline oil which is far too much for a vessel of this size to actually be holding, which has made us suspicious that she's carrying excess oil, or sanction busting," Royal Navy Leiutenant Brian Park said.

A sample of the oil was taken from one of the tanks for chemical analysis. It should reveal exactly which Iraqi oil-field the cargo comes from.

An inspection of the ship's charts uncovered a hastily rubbed out pencil line showing the likely smuggling route which hugged the Iraqi and Iranian coasts and then crossed the Gulf to Dubai.

Caught red-handed

The master of the tug, whose ship was likely to be confiscated, knew he had been caught red-handed.

"Didn't you know there were warships out to sea that were going to stop you?," I asked him.

"No I did not know sir," he replied.

So heavy with oil it was taking on water

"So this came as a bit of surprise?"

"Affirmative on that," he said.

This incident of smuggling was about as blatant as it gets. The tug was laden to the gunnels with oil which it was trying to smuggle out of Iraq under cover of darkness.

The oil would have been sold further down the Gulf, with the profits going into Saddam Hussein's pocket.

Effective crackdown

Nearby Kuwaiti and American warships were also involved in the crackdown on sanctions. By sending helicopters and small boats closer to Iraq than ever before, they reckon to have cut oil smuggling to a tenth of previous levels.

US warships are also on patrol

The only Iraqi boats we could see in the area were a handful of dhows. We also saw a container ship bringing in food that Iraq is allowed to exchange for oil under a UN programme - a legitimate trade the Cumberland also monitors.

"We can't force Saddam Hussein to feed his people, although it's clear he has the wherewithal to do so. What we can do is prevent him from exchanging oil for luxury goods and the constituent parts for weapons of mass destruction," said Captain Alan Richards, Commander of HMS Cumberland.