Michael Huether, from the German Institute for Economic Research, said British officials “haven’t achieved anything” in negotiations so far.
In a shock attack, he claimed they lack “strategic skills” and are unsure whether to pursue a hard or soft Brexit after leaving the EU.
The economist said: “All they have agreed is to negotiate in two parts: first the separation agreement, and then the agreement for a future partnership.
“This is the truly disturbing thing. We are way past one year after the Brexit vote and, in fact, the British government don’t seem to be well-positioned.
“They are not in agreement about what kind of Brexit it should be at the end of the day.
“Because first it was a hard Brexit, then it was said that no deal is better than a bad deal. Now they are slowly realising any deal is better than a bad one.”
Mr Huether also claimed the UK’s economy would “suffer” after it quits the EU, while Germany would experience no such problems.
He told TV station N-TV: “In the last few years we have learnt that if there are economic difficulties, then the German economy can adapt relatively quickly.
“Germany’s industry-services sector combination is positioned for this. This is not the case with the British economy.
“The industry there has a share of 10 per cent, while ours has 23 per cent.
“And then there is the central topic of financial markets. I believe Germany can deal with the topic.
“The British will suffer in the end, especially all those in the rural areas, who have mostly voted in favour of Brexit.
“There is no way to see how this can be compensated for economically.”
The third round of Brexit negotiations began in Brussels this afternoon.
EU official Michel Barnier suggested the UK’s demands were unclear, saying: “We need UK papers that are clear in order to have constructive negotiations.
“And the sooner we remove the ambiguity, the sooner we will be in a position to discuss the future relationship and a transitional period.
Brexit Secretary David Davis hit back, saying British position papers were sufficiently detailed and were “the products of hard work and detailed thinking”.
Additional reporting by Monika Pallenberg