EPA
Mark Rutte told the BBC the UK, outside the EU, would not be "big enough" to play a role on the world stage.
He also said the withdrawal agreement could not be renegotiated by the autumn.
Changes could be made to the political declaration if the UK tells the EU how it wants to deal with the Irish border, he said.
However, he added there was "no point" in further negotiations, unless the UK changed its red lines.
Responding to Mr Rutte's comments, ex-Brexit Secretary David Davis said Boris Johnson - who he is now supporting to be Tory leader - said: "Boris believes we have got a great future."
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The political declaration sets out the future relationship between the UK and the EU. It is a separate document from the withdrawal agreement which sets out the terms of the UK's departure from the EU.
The political declaration covers arrangements for the Irish border - stating that technology and alternative arrangements would be considered to keep the Irish border open with no physical infrastructure (eg border posts).
The backstop is a position of last resort, to maintain a seamless border on the island of Ireland in the event that the UK leaves the EU without securing an all-encompassing deal.
Speaking to BBC's Radio 4 Today programme, Mr Rutte expressed his opposition to Brexit.
"I hate Brexit from every angle, I hate no-deal Brexit from every angle," he said.
Asked about a second referendum he said: "I've stopped dreaming - we have to avoid a hard Brexit now."
Outside the EU, the UK will "not be big enough to have an important position on the world's stage", he said.
Responding to suggestions the EU could agree a transition period in the event of a no deal scenario, he said: "Hard Brexit is hard Brexit, I don't see how you can sweeten it."
He warned against implementing a time limit on the Irish backstop - as some Tory leadership candidates have proposed.
"That would mean a hard border, and that would mean an end to the Good Friday Agreement and back to the Troubles," he said.
https://www.bbc.com/news/uk-politics-48703043
