Motorists will face "untenable" delays on the Tullamarine Freeway in the next 10 years and there must be an urgent plan for an airport rail link to prevent gridlock, the Andrews government has been warned.

Congestion on the road is set to become so severe that Melbourne Airport has told the state government it "risks slowing Victoria's economic growth in the medium term".

Outbound traffic on the Tullamarine Freeway. Credit:Penny Stephens

The warning flies in the face of a decision by the government's top infrastructure adviser to effectively place the much-discussed rail link on the back-burner.

In its draft 30-year strategy, Infrastructure Victoria said the link would not be needed for up to three decades, suggesting the Sky Bus service could fill the gap in the interim.