May and her government have repeatedly said membership of a customs union would prevent it having an independent trade policy - something they have promoted as one of the main economic benefits of leaving the EU. I think you would need to have a time limit.
With a vote due on February 14, May will ask Parliament to reaffirm its desire to remove the contentious Irish backstop clause from the Withdrawal Agreement, according to an official, who asked not to be identified.
She said it was possible to pass bills "quite quickly" with "goodwill" from the Commons and Lords, but added: "It's just not possible to say how quickly it could be done, but obviously it depends on the way in which there is adequate debate on the meaningful vote and that's what the Prime Minister is determined to do".
She did not reject any of his conditions outright in her reply.
'Non-EU members do not have a say in EU trade policy so to pretend that you could do so is a unsafe delusion, ' he said.
The Labour MP said that Parliament will have to intervene with a "hard stop" over concerns Mrs May will attempt to leave MPs with a choice of accepting her deal or crashing out of the European Union without any agreement.
Shadow Brexit secretary Keir Starmer said his "key question" about Mrs May's response was: "Is she prepared to move her red lines and find a consensus?"
"It's clear from our side that we are not going to reopen the withdrawal agreement but we will continue our discussion in the coming days", Barnier said afterward.
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And I certainly don't see many defences keeping them quiet at the other end. "The manager called me and said 'do you think you can play?'".
"It is very clear from the European Union that non-EU members do not have a say in EU trade policy so to pretend that you could do so is a unsafe delusion".
She also questioned whether the call for "frictionless" trade would mean reneging on Labour's commitment to end free movement. "I think you'll find that the predicted slowdown in a number of European economies is not disconnected from the slowdown, for example, in China", Fox told a news conference in the Swiss capital.
"We shouldn't be put in a position where the clock is run down and the prime minister says it's either my deal or even worse".
She said the government needed to "get serious" about policy on a customs union and guarantee to "legislate for the protections around workers' rights".
There appears little prospect of an imminent breakthrough with Brussels, and Mrs May might not bring her deal back for a decisive vote this month.
In their letter to Mrs May, the group describing themselves as "patriotic, pragmatic Conservatives" said it the United Kingdom is "not ready to leave" on the scheduled date of 29 March.
May is already due to update parliament on her progress towards a deal on Wednesday and then on Thursday to give parliament a chance to express their opinion.
In the event of a no-deal Brexit, these would need to be completed by 29 March, an ambition that many in British business now believe to be nearly impossible.