Britain will not remain in the customs union during the transitional period planned for immediately after it leaves the European Union, two leading cabinet ministers declared on Sunday.

Liam Fox, the international trade secretary, and Philip Hammond, the chancellor, made the declaration in a joint article for the Sunday Telegraph intended to quash speculation that the cabinet is divided over how to implement Brexit and what will happen during the transitional period – or implementation phase, as ministers call it.

The article shows that Fox has won at least one internal cabinet battle over Brexit because it says the UK will be “outside the customs union” during the transition and that it “will be a ‘third country’ not party to the EU treaties”.

This means that the UK will be free to conclude trade deals with non-EU countries from the moment it leaves in March 2019, and that it will not have to wait until the transition is over, possibly three years later.

Hammond said last month that “many things will look similar” during the period and there was speculation he was pushing for an arrangement that would in effect keep the UK in the customs union, and therefore unable to sign bilateral trade deals with non-EU countries, in the period immediately after Brexit.

Fox, who was alarmed at the prospect of heading an international trade department unable to strike trade deals, admitted in July that there was “still a discussion to be had” on that issue.

Fox and Hammond are seen as the most pro-leave and pro-remain ministers in the cabinet and the Sunday Telegraph article shows that, on some issues relating to the transitional period, consensus has now been achieved.

They write: “We respect the will of the British people – in March 2019 the United Kingdom will leave the European Union.

“We will leave the customs union and be free to negotiate the best trade deals around the world as an independent, open, trading nation.

“We will leave the single market, because there was a vote for change on June 23rd and that is what we will deliver ...

“When we’ve left the customs union, we will build up on [relationships with other countries] by negotiating as an independent nation with the freedom to sign bilateral free trade agreements.”

Earlier this summer, Fox made his own concession over Brexit, accepting that a transitional period lasting two or three years might be necessary. Previously he had been downplaying the need for such a lengthy interim stage.

Pro-leave campaigners fear that the arrangements in place during a transitional phase could end up becoming permanent, leaving the UK with a relationship with the EU that would feel like associate membership.

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In their article Fox and Hammond insist this will not happen. They write: “We want our economy to remain strong and vibrant through this period of change. That means businesses need to have confidence that there will not be a cliff-edge when we leave the EU in just over 20 months’ time.

“That is why we believe a time-limited interim period will be important to further our national interest and give business greater certainty – but it cannot be indefinite; it cannot be a back door to staying in the EU.”

They go on: “We are also clear that during this period our borders must continue to operate smoothly; goods bought on the internet must still cross borders; businesses must still be able to supply their customers across the EU and our innovative, world-leading companies must be able to hire the talent they need, including from within the EU.”

The article does not explain how these aims will be achieved. It also does not say how long a transitional period would last, or what payments the UK would continue to make to the EU while it was in place.

Fox and Hammond’s declaration came as the prime minister, Theresa May, prepares to return to work this week after her walking holiday. The government is also planning to publish this week some detailed Brexit position papers, including one covering the border between Northern Ireland and Ireland, before a fresh round of talks between the UK and EU Brexit negotiators later this month.