Prime Minister Theresa May's claim that the UK's "best days lie ahead" was almost immediately dismissed by Lord Michael Heseltine on Wednesday (29 March).
The Conservative peer and former deputy prime minister told IBTimes UK that Brexit would diminish the UK's standing on the world stage.
"I don't [agree with May's statement]. I think this is a serious diminishment in British power and influence. It spits in the wind of everything that every Conservative prime minister, including this one, have argued in my lifetime," Heseltine said.
"I hope very much that events unfold that Britain changes its mind before the final end of the negotiations."
The pro-EU Tory made the remarks just hours after the UK's top diplomat to the EU, Sir Tim Barrow, hand-delivered May's Article 50 notification letter to EU Council President Donald Tusk.
The move triggered the start of the two-year-long divorce talks between the EU and UK. "We can look forward with optimism and hope – and to believe in the enduring power of the British spirit," May told MPs.
"I choose to believe in Britain and that our best days lie ahead. And I do so because I am confident that we have the vision and the plan to use this moment to build a better Britain."
But Heseltine, 84, said the "real interest" will come with the EU response to the prime minister, with a Brexit summit of the 27 other EU leaders planned for 29 April. "They're now the ones who will determine what the deal will be," he warned.
The peer added: "A very significant number – 48% of those who voted – feel that they have lost their voice [and that] there's nobody arguing, there is nobody listening. I will respond to media requests to do something to balance that position."