One of the architects of the AUKUS submarine pact has dismissed assurances given by Donald Trump about the $368-billion deal's future, warning that promises from the notoriously unpredictable US president are worthless.

The federal government is celebrating the US president's emphatic backing of the vastly ambitious submarine plan, saying his statements should kill off speculation about its future.

But a former British defence secretary and one-time army officer, Ben Wallace, has told the ABC the deal is by no means guaranteed despite Mr Trump's apparent endorsement, suggesting the pact still has a long way to run.

"Trump is so unreliable no-one can rely on his word beyond 24 hours, so the best thing to always do with him is prepare for the worst and hope for the best," he said.

" Tru

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