The UK cannot "have its cake and eat it" in relation to Brexit, a special adviser to European Commission president Jean-Claude Juncker has said.
rish woman Catherine Day, a former secretary general of the European Commission, also said it will soon become clear the extent to which the UK will still be bound by EU rules and regulations after it leaves.
"I think this is beginning to dawn on parts of UK business. In order to sell to the European Union, European Union rules will apply," Ms Day told the Institute of International and European Affairs (IIEA).
"There will be a frustration in the British civil service and the British business community that they will not have an influence over shaping future EU policy but they will of course be bound directly by it.
"But you can't have your cake and eat it, and that's one consequence of leaving the EU."
The comments came as Mr Juncker said he told UK Prime Minister Theresa May the EU was not "in a hostile mood".
Read more: Theresa May Brexit speech: We do not want 'half-in, half-out' EU deal
"We want a fair deal with Britain and a fair deal for Britain, but a fair deal means a fair deal for the European Union too," he said in a press conference. "It will be a very, very, very difficult negotiation."
Broadly speaking, EU leaders welcomed clarity from Mrs May in Tuesday's speech over what Britain wants from Brexit talks, with German Chancellor Angela Merkel promising a united front in what will be "very intensive" negotiations.
Mrs Merkel welcomed the fact that London had accepted the free movement of EU citizens was not something it could opt out of without losing other EU rights.
"There cannot be any cherry picking by Britain in Brexit negotiations," Mrs Merkel told an economic conference, adding that access to the bloc's single market was ultimately linked to accepting the EU's four freedoms.
Ms Day told the IIEA event that the EU will not be "vengeful or punitive" in the talks, which she said would be a "long, drawn-out experience". But she branded Mrs May's ambitions unrealistic.
"The idea you can have the deal you want on sectors that are important to you and you don't have to take account of anybody else's wishes is simply not a realistic negotiating target. It may be a good opening bid but it's not a realistic negotiating position."
She said she did not believe that Britain's exit from the EU would require a referendum in Ireland, but wouldn't say whether she believed one would take place. She also said that the EU would become more "continental" after Brexit, with a focus again on the Franco-German alliance, which "may not always be to our taste".
Don't let that dreary Brexit jargon get to you
Single market: Begun in January 1993 by EU Commission president Jacques Delors. It provides for the so-called 'Four Freedoms' within the 28 member states of the European Union. These are free movement of capital, goods, services and people.
The UK wants to quit the single market mainly because it wants to control migration from some EU states. This affects the free movement of people. It also wants to end the EU Court of Justice's writ in the UK. The EU Court adjudicates single market disputes.
EU customs union: This is a segment of the single market and about free movement of goods. It was a cornerstone of the trading bloc from early on.
The 28 EU member states allow goods trade between them without tariffs or border taxes. The customs union also involves common tariffs and taxes on goods coming from markets outside the EU.
EU trade deals with other markets are also negotiated by Brussels on behalf of the entire bloc. Member states are not allowed to do trade deals of their own. That is why Britain wants to end full customs union membership and do deals themselves across the globe.
Common travel area: Irish and British citizens have circulated freely on the two islands since the 1920s. The 1993 EU single market, and the paramilitary ceasefires 20 years ago, ended border controls in this country.
Both Dublin and London want this to continue post Brexit. But Mrs May's move to leave the single market and customs union makes this very difficult indeed.