Theresa May has been warned that negotiating a new trade deal with the European Union within two years of triggering Article 50 will be "quite a laborious endeavour".
German foreign minister Sigmar Gabriel cast doubt on the Prime Minister's view that the UK's future trading relationship with the bloc could be resolved at the same time as sorting out Britain's exit.
Mr Gabriel instead suggested both sides may have to be content with getting as far as they can within two years, rather than sealing the deal within that timeframe.
Liberal Democrat leader Tim Farron said Mr Gabriel's comments in an interview with The Independent "show a level of awareness that has so far escaped our government".
Germany's chancellor Angela Merkel and French president Francois Hollande have insisted the terms of the separation - including the so-called "divorce bill" - must be settled before they can move on to discuss a new trade relationship.
That puts them directly at odds with Theresa May, who said in her formal Article 50 letter to European Council president Donald Tusk that she wanted the two negotiations to proceed in parallel.
Mr Gabriel said: "We are as keen as the UK is to complete both agreements as soon as possible.
"First, we have to get things right on the orderly withdrawal, then we will want to talk as soon as possible about our future relationship and get as far as we can within the next two years.
"However, I don't want to speculate on time frames at a time when negotiations have not even started yet.
"Both sides must recognise that an agreement on a wide-ranging partnership will be quite a laborious endeavour."
Mr Gabriel, who is German vice-chancellor as well as foreign minister, said the UK and the EU "should strive to secure an orderly withdrawal as their first order of business".
Mr Gabriel added that it was "in our shared interest to maintain close security co-operation" after Brexit, as well as calling for an early guarantee on rights for both EU and UK citizens who have built up a life abroad, as another priority.
He added: "We often hear from foreigners who reside in the UK and who are very concerned about Brexit and its consequences, not only for their rights but also for the general climate towards foreigners. This concerns me."
Mr Farron said: "These remarks show a level of awareness that has so far escaped our government.
"The negotiations ahead are going to be complex and challenging.
"It is not clear what 'divorce payment' Britain may be asked to pay, it is not clear what sort of deal Theresa May is pursuing, let alone what may be reached, and we are now starting to see just how many spanners could be thrown in the works to turn this around within two years."