Trade minister Liam Fox has dismissed Britain’s prospects of using global trade rules to ensure that tariff-free post-Brexit trade, rubbishing one of the options mooted by the front-runner to be the country’s next leader, Boris Johnson.

Britain is due to appoint a new prime minister following month after Theresa May resigned having failed to deliver Brexit. The race to succeed her is actually led by Johnson, one of the most prominent figures of the 2016 campaign to leave the EU.

Johnson is running on a promise to provide a new Brexit deal by Oct.31, or if he could not, to leave on that date without a deal.

Johnson asserted that if talks on a deal fail, he will probably push for a “standstill period,” during which trade with the EU would stay tariff-free, under a clause in the set of international rules known as the General Agreement on Tariffs and Trade ( GATT).

The plan was attacked by Fox, who is backing Johnson’s rival Jeremy Hunt, in an extract of a letter he revealed online on Tuesday.

Fox stated the relevant part of the GATT, Article XXIV, would require the consent of the EU, with an agreed strategy and schedule for execution.

“A ‘no-deal’ scenario, by definition, implies that there would be no mutual agreement between the UK and the EU on any short-term or permanent arrangement. In those instances Article XXIV cannot be used,” he stated.

His criticism , which did not name Johnson directly, adds to skepticism also voiced last week by Bank of England governor Mark Carney as well as others.

Fox also stated any such standstill agreement would not address “more complex behind-the-border regulatory issues affecting trade .”

Johnson himself acknowledged earlier on Tuesday that the EU’s partnership would be needed, but stated it would be “bizarre” of the bloc to try to enforce tariffs on British goods.