The complexity of the UK’s efforts to disentangle itself from the European Union has been made clear by the foreign secretary, Philip Hammond, who said the process might take as long as six years to complete and the possibility of signing bilateral trade deals in the interim may be limited.

Hammond was speaking in the House of Commons on Tuesday as he came under pressure to explain how the Brexit process might work. An opponent of Brexit, Hammond said during the referendum campaign that the process of leaving the EU might take longer than the second world war, and on Tuesday he did not back track from this assessment.

“The concern is this,” he said. “If a future treaty between the UK and the EU 27 is deemed to be a mixed competence, it will have to be ratified by 27 national parliaments. I think I am right in saying the shortest time in which that has been done in any EU treaty is just under four years, and that is after taking into account the time it has taken to negotiate.”

A mixed agreement is one that covers subject matters over which both the EU and a member state has responsibility. Lawyers would treat the agreement as mixed if, for instance, it covered not just trade – a matter of exclusively EU responsibility – but also common foreign and security policy, for which member states are also liable.

Hammond’s assessment suggests that the UK might take two years from this winter – assuming that is when article 50 is triggered – to negotiate an exit agreement, and then wait a further four years for the agreement to be ratified. The flexibility of the UK in the period between the exit agreement and the ratification process is likely to be subject to dispute.

Hammond also underlined the difficulties the UK might face in negotiating new bilateral trade deals while it is still part of the EU. He told MPs: “Until we have served an article 50 notice, we remain a full, participating member of the EU and our ability to negotiate new trade agreements is restricted by the continued application of EU law until we have negotiated our exit from the EU.



“We have to tread a careful path having any preliminary negotiations but remain on the right aide of our international obligations.”

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The Foreign Office minister Hugo Swire added: “We are aware that we do need to trade and recruit more trade negotiators. We are still members of the EU until the end of the end of the negotiations have taken place.” But he added: “There is no reason not to have exploratory talks, which we have started to do.”

Hammond insisted his warnings about the complexities of the Brexit process did not imply a Foreign Office desire to backtrack on the referendum result. “The next PM has made very clear that Brexit means Brexit,” he said. “We will be negotiating our exit from the EU but we will also be seeking to negotiate an agreement between the UK and the EU 27 to regulate our trade and other relationships with the EU.”

The government will be consulting widely with Wales, Northern Ireland, Scotland and the mayor of London on the terms of the UK’s exit from the EU. Hammond did not, however, confer with his Labour counterpart Emily Thornberry on whether the Labour party would be formally consulted.