HOUSTON (Reuters) - BP Plc said it will begin pressure tests at its undersea oil leak in the Gulf of Mexico by midday on Tuesday after it placed a new containment cap and seal on the wellhead late on Monday.

Kent Wells, senior vice president of exploration and production, told reporters that BP and government scientists would monitor the six- to 48-hour well integrity test at “very minute intervals.”

If the test progresses as hoped, BP said no oil will flow from the well for the first time since a rig being drilled for BP by Transocean Ltd sank days after an explosion on April 20 that killed 11 workers.

Wells said high pressures would indicate the wellbore is intact after the blowout but low pressures would be a sign that oil and gas are leaking out the sides.

“Everybody hope and pray we see high pressures here,” Wells said.

While the cap could contain the flow, Wells said relief wells remain the sole means to permanently kill the leak. The first of those wells has another 30 feet to drill before inserting pipe and preparing to intercept the blown-out well by the end of July, Wells said.