Workers have eased a giant concrete-and-steel box into the Gulf of Mexico, starting the long process of lowering the contraption over the blown-out oil well at the bottom of the sea.

The mission is an untested bid to capture most of the gushing crude and avert a wider environmental disaster.

The 100-tonne containment vessel is designed to collect as much as 85 per cent of the oil spewing into the Gulf and funnel it up to a tanker.

It could take several hours to lower it into place, after which a steel pipe will be installed between the top of the box and the tanker.

The whole structure could be operating by Sunday. BP spokesman David Nicholas said that the process was complicated and they could not guarantee that it would be successful.

The well blew open on April 20 when the Deepwater Horizon drilling platform exploded 50 miles out in the Gulf of Mexico, killing 11 workers.

It has been spewing an estimated 200-thousand gallons a day in the nation's biggest oil spill since the Exxon Valdez disaster in Alaska in 1989.