BP Plc on Tuesday delayed a critical test that will determine if it can close a cap atop its ruptured Macondo well that has leaked oil into the Gulf of Mexico for the last 12 weeks.

The British energy giant and U.S. authorities said they were postponing the test that had been scheduled for Tuesday to establish whether the well can withstand the pressure caused by closing the cap at the wellhead.

“We decided that the process may benefit from additional analysis that will be performed tonight and tomorrow,” retired Coast Guard Admiral Thad Allen, who is overseeing the U.S. response to the spill, said in a statement.

The tests, due to last between six and 48 hours, had been scheduled for late Tuesday on BP’s newly installed “capping stack,” which has a better seal than the last cap placed on the well and aims to stop oil from spewing out of the failed blowout preventer.