The Brexit Supervisory Group (BSG) of the European Parliament said the proposal to replace the controversial Irish border backstop do not come “even remotely” close to a compromise, and cannot get the support of MEPs.
The committee is made up of experts and senior members of the party groups in the European Parliament, which must approve any Brexit deal.
"The UK proposals do not match even remotely what was agreed as a sufficient compromise in the backstop," the group said.
They were particularly critical of the PM's proposals to replace the Irish backstop by keeping Northern Ireland tied to single market rules for trade in goods, while leaving the customs union.
Mr Johnson's plans "do not address the real issues" over the emergency mechanism to prevent a hard border returning to the island of Ireland, the group said.
The MEPs said the proposals could jeopardise the Good Friday peace agreement and the integrity of the single market.
They highlighted three concerns as a lack of clarity over where and how customs checks would take place, the details only being fully worked out in the transition period, and the right of consent being offered to the Northern Irish Assembly.
They concluded: "The BSG has grave concerns about the UK proposal, as tabled.
"Safeguarding peace and stability on the island of Ireland, protection of citizens and EU's legal order has to be the main focus of any deal."
On Thursday, the European Council President Donald Tusk added his voice to the cool reception for the plan in Brussels, saying after a conversation with Mr Johnson that "we remain open but still unconvinced".
And the Irish government also hardened its language on the proposals, with Irish Premier Leo Varadkar saying the Brexit plan "falls short in a number of aspects".
His deputy Simon Coveney told the Irish parliament that "if that is the final proposal, there will be no deal".
Speaking on a visit to Stockholm alongside the Swedish Prime Minister, who reaffirmed the EU’s solidarity with Ireland, Mr Varadkar said he could not fully understand how the UK envisages Northern Ireland and Ireland operating under different customs regimes without the need for customs posts.
Mr Coveney raised concerns about the plan to give Stormont a veto on the plan because its voting structures mean a bloc of MLAs from either the nationalist or unionist community - which includes Mr Johnson's DUP allies - can block certain decisions, even if a majority of members back them.
"We cannot support any proposal that suggests that one party or indeed a minority in Northern Ireland could make the decision for the majority in terms of how these proposals would be implemented in the future," he said.