The PM is on a whistle-stop tour of Europe trying to gather support for changes he wants before holding the UK’s EU membership referendum.
He urged his fellow EU leaders to be “flexible and imaginative” Reports BBC.
But one of Germany’s most senior businessmen said Europe should refuse to negotiate with him.
Asked how far Ms Merkel should go in accommodating Britain’s requests, he said: “Our recommendation is not to deal under such circumstances.”
Mr Cameron is having breakfast with Polish Prime Minister Ewa Kopacz before travelling to Berlin.
Speaking ahead of the visit Rafal Trzaskowski, Poland’s Minister for European Affairs, told the BBC they would listen but that any treaty change would be very difficult.
“The European Union needs Great Britain, the discussion will not be easy,” he said.
“As far as treaty changes are concerned or the introduction of discriminatory measures, that would be a red line for Poland.”
He added: “If every country comes with a shopping list to change European Union policies, that will be the end of the European construction, it will simply implode.”
Mr Cameron, who is also visiting Poland on Friday, has not set out in full detail the reforms he is pushing for, but they will include tougher rules to prevent migrants claiming benefits.
He also wants safeguards to protect the City of London in the event of closer eurozone integration and an exemption for Britain from the EU drive for “ever closer union”.
Securing the backing of Ms Merkel, who leads Europe’s largest economy, will be crucial to his chances of negotiation success.
She has previously said she wanted the UK in a “strong and successful Europe” while stressing the importance of the free movement principle, which has seen a rise in the number of EU migrants coming to the UK.