BERLIN (Reuters) - The European Union must prevent a reversal of integration if Britain votes to leave the 28-member bloc in its referendum this week, German Foreign Minister Frank-Walter Steinmeier was quoted as saying on Monday.

"Should the British opt for a withdrawal from the EU, then we cannot continue just doing business as usual with 28 minus one the next day," Steinmeier said in an interview published by several local newspapers.

A British exit from the EU, or Brexit, would severely damage the European project and the remaining member states would then have to do everything they could to prevent a reversal of the integration process, he said.

"We in Europe bear a joint responsibility for ensuring that we do not fall back into a nationalism of single states," Steinmeier said.

Campaigning for the referendum resumed on Sunday after a three-day hiatus prompted by the murder of Jo Cox, a Labour Party lawmaker and ardent pro-European, by a man who gave his name as "death to traitors, freedom for Britain" when he appeared in court on Saturday.

Three opinion polls ahead of the vote showed the 'Remain' camp recovering some momentum although the overall picture remained one of an evenly split electorate.

Both sides sought to adopt a more measured style on Sunday, paying their respects to Cox but sticking closely to the immigration versus economy debate that has defined the campaign.

Steinmeier's comments came after Finance Minister Wolfgang Schaeuble made an impassioned plea for a stronger, unified European Union, warning that no country in Europe would be able to cope with the challenges of the 21st century on its own.

(Reporting by Michael Nienaber; Editing by Tom Heneghan)