The Brexit deal sealed by British PM Theresa May and the EU is the "best and only solution" for an orderly withdrawal, German foreign minister Heiko Maas said Wednesday, stressing the bloc won't leave Ireland isolated.

"Our position is clear: the withdrawal agreement is the best and only solution for an orderly exit," he told the Funke media group, after British MPs voted for May to renegotiate the deal.

MPs late Tuesday voted through an amendment saying they would only support a divorce deal if its controversial "backstop" clause to keep the Irish border open was removed.

Read: May seeks to revise Brexit deal but EU rules out changes

Maas said it remained unclear what the British government wants amended.

"It must now quickly say what it wants because time is short," said Maas.

He stressed however that Germany and the EU stood firmly behind Ireland on the issue.

"We won't allow Ireland to be isolated on this question," he said.

Brexit hardliners from May's Conservative party think the backstop - created to keep the border open with Ireland - could see Britain indefinitely tied to EU trade rules.

The winning amendment calls for the backstop to be replaced with "alternative arrangements to avoid a hard border" - vague wording that did not pin May to any specific approach.