The latest version of the document on the future UK-EU relationship does not offer enough certainty, Wales' First Minister Carwyn Jones has said.
He and his team were briefed on the future relations declaration before meeting Theresa May on Wednesday.
The prime minister then travelled to Brussels as negotiators scramble to finalise a Brexit deal by Sunday.
Mr Jones said his meeting with Mrs May was "better than I expected" but there were areas where more work was needed.
They held the discussion in her office in the House of Commons.
The UK and EU negotiators reached agreement last week on the withdrawal deal - covering issues such as the £39bn 'divorce bill', citizens' rights, and the Irish border - but discussions continue on the joint political declaration on future relations.
Asked if his officials are happy with the latest future relations document, Mr Jones said: "Well, no because there's no certainty there.
"It's essential that we move on from a statement towards something that is far more certain and solid in terms of how the relationship will work in the future, how much access we'll have to the single market, and, if we're not in the customs union, how will that customs relationship work?"
The prime minister has been under intense pressure since reaching a draft Brexit agreement with the EU.
Given the opposition from within her own Conservative Party and from the opposition benches, there have been widespread predictions that Mrs May will struggle to get enough support in the House of Commons for her Brexit plans.
However, Carwyn Jones said: "She doesn't look like somebody under pressure, I must say. You could think that everything's fine when talking to her.
"She gave the impression of someone who is confident [that the vote in the Commons will pass] but I don't think that impression reflects reality," he added.
It had been expected that the Welsh Assembly would hold a special meeting on Thursday 29 November to debate and vote on the UK Government's Brexit deal.
But BBC Wales Brexit correspondent James Williams said that the Welsh Government dropped the proposal. It was thought some other parties were not happy with next Thursday.
The debate is now expected to take place in December.
Asked if the Welsh Government motion for the debate was likely to reject the prime minister's deal, Mr Jones said: "Yes, because it doesn't go far enough as it stands, that's my issue with it.
"It doesn't give us the certainty we need and there's work to be done to reach that level of certainty."
Following Wednesday's meeting between Mrs May and the first minister, a Downing Street spokesperson said: "They agreed that the priority remained to finalise a deal that will provide certainty and stability for people and businesses across the whole of the UK, including Wales.
"The prime minister set out how the deal we are negotiating with the EU would see the creation of a free trade area for goods and secure the rights of EU citizens in the UK and UK nationals in the EU.
"It would mean close cooperation across issues such as travel, transport and energy, protecting tourism, jobs and livelihoods for people in Wales and the whole of the UK.
"The prime minister was clear that the negotiated deal will deliver on the result of the referendum, and protect the integrity of our United Kingdom," the spokesperson added.
By BBC Wales political editor Felicity Evans
We're in the end game now and there's not much time left for the Welsh Government to try to influence Theresa May.
So how much leverage does Carwyn Jones have? He hass already been criticised by Plaid Cymru for being too conciliatory, after he struck a deal on devolved powers coming back from Brussels while the Scottish Government is fighting a court case.
He says his deal is a good one, providing some certainty at an uncertain time. They say he is not pushing hard enough.
The first minister also argues the prime minister's approach to Brexit has softened over the last two years. But she is now hemmed in by conflicting demands. Brinksmanship is her strategy.
After the Brussels summit this weekend, we will know more about whether negotiations on the future trading relationship with the EU has moved in the direction Carwyn Jones wants.
But even then, the uncertainty remains over whether any sort of deal will be accepted in Parliament.