If the UK votes to leave the EU next week, the move could ultimately lead to the bloc's disintegration, Germany's foreign minister has warned.
Frank-Walter Steinmeier was speaking near Berlin after talks with his French counterpart, Jean-Marc Ayrault.
Mr Steinmeier warned against "a nationalism that pits one European state against another".
Leave campaigners in the UK have previously dismissed similar warnings as scaremongering.
The referendum on whether to leave or remain will be held on 23 June.
Germany is the UK's biggest trade partner in Europe.
"A vote to leave would shake the union," Mr Steinmeier said at a joint news conference in Brandenburg.
"It would not just carry on as 28 [members] minus one. It would require concerted efforts to ensure that the union holds together and that a decades-long, successful integration effort does not end in disintegration."
Sweden's foreign minister Margot Walstrom made similar warnings earlier this week.
Ms Walstrom, who served as EU Commissioner for 10 years, said a British exit could trigger referendums in other states.
She said even if Britain voted to remain in the EU, other countries might seek to renegotiate the terms of their membership.
"The spill-over effect will be unfortunately felt, deeply felt," she said. "It would be bad either way."
Mr Ayrault said the EU would keep evolving with the times.
"Europe can't be static, it must keep moving," he said. "Today it faces contradictions, slow-downs, difficulties, anxieties and fears. We want to give Europe a new dynamism."