A terms of reference document setting out the rules for both sides also states: "For both parties the default is transparency".
David Davis, the UK Secretary of State for Leaving the European Union, and the bloc's Chief Negotiator for Brexit Michel Barnier met in Brussels for the first day of talks on Monday morning, as the two parties try to disentangle Britain from Europe.
The EU had demanded early agreement on the rights of EU nationals living in the United Kingdom, the Brexit divorce bill and the Northern Irish border, before any discussions could begin on a trade deal.
"We believe it is necessary to agree the terms of our future partnership alongside those of our withdrawal from the European Union".
However the survey also showed a possible change of heart about Brexit: 51% said they would vote "Remain" in a second referendum, with 49% supporting "Leave".
"We must think about the deep and meaningful partnership with Europe", he said.
Davis said the talks were off to a "promising start" and denied that Britain had caved in on the sequencing of the talks.
Mr Davis rejected the suggestion this amounted to a climbdown by the United Kingdom, arguing that trade negotiations would take place in parallel with those on the arrangements for withdrawal at a later stage.
Brexit talks under way in the Commission's Berlaymont building on Monday. We keep hearing that they don't want a 'Norway model, ' they don't want a 'Swiss model, ' they want to leave the customs union, the internal market, they want to limit migration.
Anxious by immigration and loss of sovereignty, Britons voted a year ago to end their country's four-decade membership in the 28-country bloc - the first nation ever to do so.More news: With the clock ticking, UK, EU play up chance of deal
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"In the first step, we will deal with the most pressing issues".
He added: "It's not as if Europe is leaving Britain; Britain wants to leave the EU". Britain insists that it must regain the right to control immigration and end free movement from other European Union countries into Britain.
The UK has quietly backed down from its earlier insistence on negotiating the terms of its withdrawal from the European Union and a new trade deal simultaneously.
Merkel spoke after meeting with Romanian President Klaus Iohannis, who also stressed the 27 European Union countries' unity.
"So we have got a very sensitive political context, a very clear objective which is to preserve all the dimensions of the Good Friday Agreement and we have an bad lot of work to do - bilaterally, and also in coordination with the Dublin government on my side, so that we come up with imaginative and concrete solutions along the lines I have described, particularly taking into account the single market".
"For me, it is above all about the EU27 proceeding together and listening carefully to Britain's wishes and expectations", Merkel said after meeting Romanian President Klaus Iohannis.
Many in Brussels fear that London has no real strategy, with May under pressure at home, still trying to close a deal with a conservative Northern Ireland party to stay in power, and facing criticism for her handling of the aftermath of a devastating tower block fire.
Mr. Barnier has said he hopes to conclude the talks by October 2018, leaving six months to ratify the deal.