Talks on Britain’s withdrawal from the EU may be the most complicated negotiation of all time, the Brexit secretary has said.

In a sign of the difficulties already emerging, David Davis said he had been “given 180-degree opposite opinions on some things” in terms of legal advice.

The minister for the newly created Department for Exiting the EU told a House of Lords select committee that not much could be revealed about how the UK intends to leave until article 50 was triggered to formally notify the EU of its intention to exit the bloc.

Davis promised a set of clear aims would be published then but cautioned that the process would be difficult.

“At that point we will have some clear negotiating guidelines,” he said. “Before article 50 is triggered [it] will be a rather frustrating time as we won’t be saying much. After, I expect it to be a more open process.”

He added: “It may be the most complicated negotiation of all time. By comparison, Schleswig-Holstein [a 19th-century political conundrum] is an O-level question.”

Davis stressed several times that overall responsibility for the negotiations lay with the prime minister.

Asked how the talks would be carried out in the EU, he said: “We haven’t really had time to talk that through in any detail. Nobody is really going to Brussels until [Theresa May] has been at European council in October and from then on we think the cascade will start.”

Davis said he thought there was a tug-of-war going on in Brussels over who would be responsible for negotiating with the UK, but the European commission would ultimately have to be the servant of the European council when it came to the final discussions.

He said he would be as open as possible with MPs about the discussion, but added that some things would need to be kept secret even from parliament.

“Clearly there is a need for parliament to be informed without giving away our negotiating position,” he said. “ [But] I may not be able to tell you everything, even in private hearings.”

Some businesses, charities and the education sector have been among those worried about the lack of information coming from the government.

Davis said the fact that universities and farmers were “fretting” about the uncertainty was a reason that the Treasury had agreed to guarantee the level of funding they currently get from the EU. He said the government would also need to address the “legitimate concerns” of the City of London.

In the face of warnings from countries such as the US and Japan about the impact on their companies investing in the UK, Davis said he would be going to businessmen to ask scientific questions, such as: “Can you please quantify this for me? Can you quantify the effect of a non-tariff barrier versus a tariff barrier? Which of this matters? Everyone will say their issues matter the most and I need to get some numbers on it.”

His comments suggest the government is actively considering the option of accepting and imposing tariff barriers in trade relations with EU countries.

Davis admitted his department was “stepping on absolutely everybody’s toes”, but claimed all of Whitehall accepted that the Brexit was an enormous and important priority.

He said the issue of leaving the EU was “the sexiest area of politics at the moment”, arguing that he could get the best brains working on the problem.