An influential grouping of conservative US lawmakers has taken aim at the Jones Act, the cabotage rule that requires US-built ships for domestic trades.

The Republican Study Committee (RSC) included a provision to "return shipping policy to the free market" as part of an alternative budget proposal that is broadly aimed at ratcheting down on government spending.

The budget proposal was revealed days after US Senator John McCain reintroduced legislation aimed at removing the US-build requirement of the Jones Act — which also requires qualifying ships to be controlled by a US company and have 75% American crew — in a move that has been overshadowed by last week's bombshell announcement that the former presidential candidate has brain cancer.