An underwater robot collided Wednesday with a cap placed atop BP PLC's leaking deep-water well in the Gulf of Mexico, temporarily slowing the company's ambitious plan to nearly double the amount of oil it captures each day at the offshore site.

The underwater collision forced BP to halt operation of the Discoverer Enterprise, the larger of two ships capturing oil where the Deepwater Horizon rig exploded on April 20, killing 11 workers. A mile-long pipe from the cap to the ship helps recovery crews siphon...