The businessman, who bankrolled the Labour Leave campaign during the EU referendum, put UK businesses on alert over a potential standoff in parliament when the final Brexit deals is voted on.
MPs and the House of Lords will get a vote on the final deal arranged with Brussels before it goes to the European Parliament for approval.
Critics, however, said the Government was holding a gun to the heads of MPs and the vote was not “meaningful”.
Remainer MPs are expected to attempt to vote down the proposed deal in an attempt to derail or even stop Britain from leaving the European Union.
Labour donor John Mills warned MPs of the dangers of blocking Brexit
The biggest danger we face is Parliament rejecting the final Brexit deal
Speaking to Express.co.uk ahead of the launch of his book ‘ Britain’s Achilles Heel – Our Uncompetitive Pound’, Mr Mills said: “I think the biggest danger that we face is the Government will negotiate a deal with the European Union and bring it back to parliament, who will reject it.
“That will leave a real standoff with no clear way ahead. I think the sort of uncertainty that would produce is the real threat to business rather than anything else.”
Mr Mills voiced concern at the level of Remain-supporting MPs sitting in the Commons but insisted their attempts to stop the European divorce would result in a ‘hard’ Brexit.
He claimed there would have to be a point at when the Government must accept a deal or the terms offered by the EU would only get worse.
Mr Mills added: “The best deal for us would be to come out of the single market, the European Economic Area, customs union, and then renegotiate a free trade deal.
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“I think that will satisfy most people in this country. I think the problem is what are we going to do if that option is not available, and we get much tougher terms after by the European Union.”
Remoaners Club: These people hate Brexit! Mon, January 16, 2017
Remainers are finding it hard to accept Brexit.
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The businessman said anything else would mean the UK still paying into the Brussels budget coffers, urging British negotiators to be open to walking away from the negotiating table if a deal isn’t emerging.
Mr Mills concluded: “At some point, I think the British Government has got to be prepared to walk away.
“I think with any negotiation if you’re not prepared to walk away, if the deal is bad enough, you will never get a good deal.”