European Parliament president Antonio Tajani said the situation was "not good" and "we need to know what the UK wants to do" as he restated Brussels' demands for a financial settlement which could be as high as Â£53 billion (60 billion euro).
Ahead of a meeting of European leaders on Thursday, Ireland's Leo Varadkar said they were still unclear about what the UK hoped to achieve in a Brexit deal.
In a sign of the economic risk caused by the uncertainty in the Brexit process, a senior City figure warned that firms would have no choice but to move jobs to other European Union countries in order to ensure they can keep trading unless progress was made.
London Stock Exchange Group Xavier Rolet suggested that jobs would be lost overseas unless details of a transitional period were agreed by the end of the year.
Mr Tajani said the UK Government was "not realistic" in its approach to the financial settlement - the so-called divorce bill - and echoed Margaret Thatcher's language when she secured the British rebate.
"We need our money back, as Mrs Thatcher said 30 or 40 years ago. This is important for us," he told BBC's Newsnight.
"We need not one euro more, not one euro less."
The figure of around Â£18 billion (20 billion euro) that Mrs May was offering is "peanuts", he said, with "the real situation" being a sum of around Â£45- Â£53 billion (50-60 billion euro).
Differences within the Cabinet were illustrated on Tuesday as Home Secretary Amber Rudd suggested that a "no deal" scenario was "unthinkable", at least on security issues, while Brexit Secretary David Davis insisted the UK had to keep open the option of walking away from the talks.
There are also divisions over how close any future relationship will be, leading Mr Tajani to say: "In the Conservative Party there are different positions. This is not good for good work in the next months."
Mr Varadkar told BBC's Spotlight: "It is very hard for us as European prime ministers to understand exactly what the UK wants the new relationship to look like."
Meanwhile, Mr Rolet said that regulatory systems that will change as a result of Brexit "cannot be replicated overnight" so businesses will simply make their own arrangements and move to the EU if they are not given sufficient notice.
"Businesses cannot risk a cliff-edge being so near before they start accelerating the migration of functions away from the UK to ensure they can participate in the global market," he wrote in the Daily Telegraph.
Mr Rolet also warned European leaders that protectionist moves to strip London of its role in euro clearing could increase the risk in financial markets.
Taking back euro trading into the Eurozone would "increase systemic financial risk" because "the global monitoring of risk would be impaired and that risk more heavily concentrated".
He said: "To those who want to dismantle rather than build on a system of global regulatory standards that protects taxpayers and reduces the cost of capital, we say: do not willingly diminish systemically important global financial market infrastructure just to make a political point."