BP has been accused of dumping industrial waste at sea after starting to drop thousands of tonnes of oil pipes in a legally protected marine wildlife zone in the Atlantic.

Confidential documents seen by the Guardian show the oil company sought approval to dump 14 pipes and control cables 120 miles west of Shetland after finishing drilling at the site.

It started dropping four days ago on to a marine protection area (MPA) after being given clearance last week by the UK’s decommissioning regulator.

The area is designated an MPA under international law because of its rare giant deep sea sponges, gravel ecosystem and ocean quahog, a very slow-growing mollusc. A type of clam, ocean quahog are one of the longest-living animals on Earth, and have been known to live for 400 to 500 years.

BP has been drilling there for 25 years, at depths of up to 600 metres, using a floating oil ship called the Petrojarl Foinaven, which is to be scrapped. The company also plans to drop all the ship’s steel mooring lines and anchors on the site, totalling 4,180 tonnes.

BP, which made £5bn in profits in the first quarter of 2022 on the back of a global surge in oil and gas prices, had originally applied to the regulator for approval to lower the 14 pipes and cables in a controlled manner.

But after a series of delays, those plans were repeatedly changed. It has now been allowed to release all the pipes where they join the ship, so they fall unguided on to the sea floor. Four have been released so far.

The riser cables are about 820 metres long and the umbilical cables up to 4.2km long; they weigh nearly 2,400 tonnes. A source with direct knowledge of the plans described the web of cables as being like a pile of “cooked spaghetti”, and said it would be immensely costly and challenging to recover all 14 cables and risers, suggesting it would cost tens of millions of pounds and need special retrieval ships and underwater vessels.

BP insists that dumping the cables will have little impact on the seabed and will still allow the pipes to be recovered later. It has given itself six years to decide whether to resume drilling at Foinaven, sell it or finally decommission it.

Dr Doug Parr, the chief scientist at Greenpeace UK, said: “The only circumstances in which any company should contemplate dumping hardware on to the seabed in an uncontrolled manner would be to save lives in an emergency. The fact that BP is proposing to do this simply out of expediency is hard to defend.”

The company said it was legally obliged to recover all the pipes and cables, and denied the uncontrolled release was intended to save money. It said it had sped up the process for safety reasons, because the summer weather-window in that exposed part of the Atlantic was narrow.

“Our plans to recover and dispose of the Foinaven risers and our commitments to minimise impact on the environment as part of our decommissioning process remain unchanged,” BP said.

“Solely due to safety considerations, our proposed method of disconnecting the risers has changed, but our plans to recover and dispose of the risers have not. However, it will still be done in a controlled and sequenced manner.”

Known as the Faroe-Shetland sponge belt, the MPA covers a deep sea channel and rift basin partly gouged out by glaciers that is up to 800 metres deep and has the only population of giant sponges in UK waters. Known to fishers as “cheese bottoms” because of their appearance, the sponges support large schools of fish and brittlestar.

BP’s application to the Offshore Petroleum Regulator for the Environment and Decommissioning (Opred) acknowledged the disposal would cause “localised, temporary disturbance of an area of seabed”. The company said only a fraction of the MPA would be affected by dropping the risers and cables, covering roughly 70sq metres.

Parr said it was far from certain the pipes and cables would be recovered. “Since the regulator has given permission for the risers simply to be dropped, with the potential for far greater impact on the seabed and making future recovery far less likely, this could set a precedent for other oil and gas companies to do the same,” he said.

A spokesperson for the Department for Business, Energy and Industrial Strategy, which includes Opred, confirmed BP was expected to recover the equipment and recycle it. “Decommissioning is undertaken in accordance with UK and international obligations, in a safe and cost-effective manner while minimising risk,” they said.

“Each case is taken on its merits and, as with every proposal, this will have been assessed comprehensively on its environmental impact.”