Chuka Umunna’s comment came as Theresa May prepared for a major speech later this month, in which she is expected to spell out her vision of Brexit and her plans to create a “truly global Britain”.
Reports suggest the Prime Minister will say Britain will pull out of the single market if the European Union fails to make concessions on freedom of movement, although No 10 sources insisted the claims were speculation.
Meanwhile, a former Whitehall boss warned that the surprise resignation of Sir Ivan Rogers as ambassador to the EU had exposed concerns about the Government’s state of readiness for withdrawal negotiations under Article 50, due to begin within less than three months.
In a letter to the Times, Lord Kerslake - head of the Civil Service from 2012-14 - said Sir Ivan’s departure “deprives the Government of a very experienced and capable European expert at a critical moment”.
And he warned: “What is equally concerning though is the manner of his departure and what it tells us about the Government’s state of readiness for what many see the biggest challenge this country has faced since the Second World War.”
Mrs May moved swiftly to draw a line under the row over Sir Ivan’s shock departure by agreeing a replacement within 36 hours of his notice to quit.
Downing Street described his successor, former Moscow ambassador Sir Tim Barrow, as a “seasoned and tough negotiator” who would help the Government make a success of Brexit.
His appointment was hailed by Brexit Secretary David Davis and Foreign Secretary Boris Johnson and welcomed by Labour, but Ukip said the role should have been given to a “committed Brexiteer”.
Ukip spokesman Gerard Batten called for a “revolutionary shake-up” of the Foreign Office to reflect public support for Brexit, adding: “This appointment is a disappointment because the last thing we need is another career diplomat wearing a Brussels jersey.”
But Mr Johnson said Sir Tim was “just the man” to secure the best deal for the UK, praising his “relentless energy, wise counsel and steadfast commitment”.
Former diplomat and European Commission official Sir Robert Cooper said the new ambassador would be faced by a “policy vacuum” when he arrives in Brussels next week.
“I think at the moment there is a policy vacuum,” Sir Robert told the BBC Radio 4 Today programme. “I think at the moment probably the atmosphere is difficult because people don’t know where they are going. You need to have a sense of direction.”
Mr Umunna also warned of a lack of direction, telling ITV’s Good Morning Britain: “Holding this referendum without any plan as to what you would do if we voted to leave was criminal, in my view.
“Well, I’m being rhetorical, it’s clearly not illegal, but it was a silly thing to do, and now what have we got? We are flailing about, no direction, no-one knows what’s happening.”