BP Plc delayed retrieval of the failed blowout preventer atop its ruptured Gulf of Mexico oil well this week because of bad weather, the top U.S. official overseeing the oil spill said on Monday.

We are in a weather hold right now, retired Coast Guard Admiral Thad Allen said, noting seas were 6 to 8 feet (1.8 to 2.4 metres) high at the site of the Macondo well off the Louisiana coast.

Allen gave the telephone briefing while aboard a rig waiting to resume drilling a relief well that is on target to intercept the Macondo well and kill the leak for good.

Drilling cannot resume until the failed blowout preventer is removed and replaced by another, so the delay in this step will postpone the relief well's work as well, Allen said.

He said bad weather is expected to last two or three days. That would push the blowout preventer switch to later this week from Tuesday or Wednesday.

We should have been ready to go right after Labour Day but obviously this will move that, Allen said of the relief well.

If for some reason the weather lays down, we will go immediately, he said.

He said the weather issues are unrelated to tropical systems that swirled above the Atlantic Ocean on Monday.

The failed blowout preventer a mile (1.6 km) beneath the water's surface is key evidence in criminal and civil investigations into the April 20 blowout that preceded an explosion aboard the Deepwater Horizon rig and killed 11 men.

The switch will preserve the evidence and ensure a working blowout preventer is atop the well to handle any pressure increases as the relief well bores in to inject mud and cement near its bottom about 13,000 feet (4,000 metres) beneath the seabed, according to Allen.

He said on Monday that the choppy seas hinder removal because rigs bob up and down and force a pipe lifting the equipment to swing back and forth like a pendulum.

(Editing by Mohammad Zargham)