BP has reported a "significant oil discovery" in the Gulf of Mexico, its first major find since the deadly rig explosion that triggered the worst environmental disaster in US history.

The company said it had hit oil at depths close to 9,150 metres (30,000ft) at its Gila prospect in the Gulf of Mexico, about 300 miles south-west of New Orleans.

The announcement marks the first big oil discovery since US regulators lifted a five-month ban on deep-water drilling in 2010 after the Macondo well blowout; it follows two finds in the Gulf in 2006 and 2009.

The discovery, whose commercial potential remains unclear, came as BP revealed a $1bn (Ł650m) write-off from its Pitanga well off the coast of Brazil, which never yielded the lucrative fossil fuels the company had hoped for. The oil company admitted it would not recover the $850m it paid to buy the Pitanga well, nor a further $230m spent on developing it.

BP said 2013 had been its most successful year for oil exploration for almost a decade: it had investigated 15 wells, making seven potentially commercial discoveries.

The company expects to spend around $4bn a year exploring and drilling new wells in the Gulf of Mexico over the next decade, a figure roughly equivalent to the sum it has set aside for clean-up costs, fines and compensation related to the disaster, which killed 11 people and released 4m barrels of oil into the sea. The final bill will not become clear until a US court judgment next year.

BP, which employs 2,300 people in the Gulf of Mexico, had seven wells in 2012, up from five in 2011, a further sign of the drilling revival in the region since the spill. While fracking and cheap gas have captured public attention, big oil companies have been moving back to the Gulf, building new rigs. A record 807 oil permits for the Gulf were issued in the first nine months of this year, up 14% on 2012, according to Bloomberg.

BP continues to fight a ban on competing for US government contracts that was imposed following the oil rig explosion, recently winning the support of the British government.

"The Gila discovery is a further sign that momentum is returning to BP's drilling operations and well execution in the Gulf of Mexico," said Richard Morrison, regional resident of BP's Gulf of Mexico business. The US oil major ConocoPhillips has a minority stake in the Gila prospect.