In an article published by The Sunday Telegraph, the two said any transition period would be "time-limited", and that Britain would not have access to the European single market or the customs union after it leaves the bloc.
LONDON (AP) - The British government is fighting back against criticism that it is divided and unprepared for Brexit, announcing it will publish a set of detailed proposals on customs arrangements, the status of the Ireland-Northern Ireland border and other issues. "That means businesses need to have confidence that there will not be a cliff-edge when we leave the European Union in just over 20 months' time", they wrote.
In what seems as an effort to lessen those fears, the ministers write: "We believe a time-limited interim period will be important to further our national interest and give business greater certainty - but it can not be indefinite; it can not be a back door to staying in the European Union".
"We are both clear that during this period the United Kingdom will be outside the single market and outside the customs union and will be a "third-country" not party to EU treaties", they said.
The joint comments by Hammond and Fox came after weeks of clashes over the UK's future outside the bloc.
This week we're expecting to find out the UK Government's position on the border.
The announcement appeared to be an attempt to mask over an apparent lack of unity within the ruling Conservative party on how to proceed with the Brexit.
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Former Prime Minister Tony Blair has also called for pro-EU politicians from all parties to unite.
Mr Miliband also described Brexit as an "unparalleled act of economic self-harm".
"We've been crystal clear that issues around our withdrawal and our future partnership are inextricably linked, and the negotiations so far have reinforced that view", a source in Britain's Brexit department said.
"We will leave the single market, because there was a vote for change on 23 June and that is what we will deliver".
"The referendum will be no excuse if the country is driven off a cliff", he declared.
The disclosure comes as Britain's Brexit secretary David Davis prepares to embark on a third round of talks with the EU's chief negotiator, Michel Barnier, in the Belgian capital at the end of August.
The next round of talks is due at the end of the month, with both sides looking for progress towards a solution to three of Brexit's thorniest problems: how much Britain should pay to leave, what rights British and European Union citizens will have, and how to manage a land border to the bloc in Ireland.