Boris Johnson said the U.K. is on the brink of a “historic mistake,” comparing Theresa May’s Brexit plan to the Titanic heading toward disaster.

To help avoid the return of checkpoints on the Irish border, May’s deal suggests the entire U.K. could remain in a customs union with the EU until a better solution is found. But Northern Ireland will also keep many of the EU’s rules — and that means added checks on goods arriving from mainland Britain.

“The Titanic springs to mind, and now is the time to point the iceberg ahead,” Johnson, the U.K.’s former Foreign Secretary, said in a speech at the Democratic Unionist Party conference in Belfast on Saturday. “This deal risks further economic and political humiliation.” His speech received a standing ovation.

Johnson called on May to “junk” the so-called backstop, which he said risks leaving the U.K. as a “satellite state.” Both the U.K. and EU say they want to avoid the backstop ever being triggered, but so far that hasn’t been enough to reassure the DUP, whose 10 lawmakers keep May in power.

The EU could yet demand the U.K. accept a “special deal on immigration”, Johnson said, seeking to raise doubts around one of the key benefits May says flow from her deal — ending freedom of movement.