Victorian taxpayers are facing a massive East West Link compensation bill – even if a court renders the contract for the $6.8 billion road invalid.

In a high-stakes political gamble, former treasurer Michael O'Brien on Thursday released a previously secret "side letter" reassuring the East West Connect consortium that it would get its money regardless of whether a valid contract exists.

The need for the extraordinary guarantee was triggered by publication of a report in The Age in September last year revealing Labor intended to dump the project if it won the election. To make that case, Labor relied on high-level legal advice to claim the contract was "unenforceable" because of a looming court case waged by the Yarra and Moreland councils.

The Age also revealed the East West Connect consortium threatened to walk away from the project through fear it would be dumped if Labor won, prompting the Department of Treasury and Finance to recommend the side letter for reassurance.