Lord Heseltine has said he is “not surprised” that leading Brexiteers in government appear not to understand what they are doing, in a thinly-veiled attack on senior Eurosceptics promoted to Theresa May’s Cabinet.

While not naming Boris Johnson, David Davis and Liam Fox, the Tory peer said it was right that those who campaigned to leave the EU be put in charge of Government departments to ensure they take responsibility for the difficult decisions ahead.

“I said at the time, you’ve got to put the Brexiteers in charge, or otherwise the right wing will never accept it. They will say if we’d have been there we’d have done it better,” he said during a discussion event at Oxford University.

“I have to say there was an element of cynicism in my judgment because I knew they wouldn’t have a clue what to do. And read today’s newspapers and you’ll see each is bickering with the other.

“They say, ‘We must tell the Europeans what we want.’ But the Europeans will tell us what we can have. We are leaving the club. The club will be the ones that say what the deal is going to be.”

Speaking shortly after the emergence of his letter to the Prime Minister in the wake of his sacking from his advisory role in government, he said: “She didn’t believe Brexit was right but perhaps she now does. If she now does she must be entitled to pursue her new found convictions. It doesn’t mean I have to join her.”

Lord Heseltine speaking in Oxford; the Tory peer was sacked by Theresa May for rebelling over Brexit

The party grandee also read from the letter he had received from Theresa May, notifying him that he had been removed from his role as a government advisor over his decision to vote against the Withdrawal of the European Union bill.

It said: “I’m sad that this important contribution had come to an end following your decision to vote against the Government on the European Union Notice of Withdrawal Bill, but I know that with your many years of experience in Parliament, you understand why that must be.”

He then read from his own letter to Theresa May, in which he said, the Prime Minister had changed her mind on Brexit, but “I have not”.

The former Cabinet minister also said he was “appalled” at the prospect of the breakup of the United Kingdom.

“I am appalled by the thought of the dismemberment of the union,” he said.

Michael Heseltine lays into Boris Johnson

“I believe the Prime Minister is quite right to stand firm. This argument that we’re not in a position to know so how can they make up their mind.

“That is exactly the question that we faced in the [EU] referendum.

“So the Prime Minister is precisely repeating the Remain argument in her relations with the Scots.”

The lifelong pro-European campaigner said he had been committed to Europe since “standing under German bombs in 1940”.

He also praised Margaret Thatcher for the “most enlightened achievement” in recognising that the UK “had to lead the process that would lead to the creation of the single market”.

Asked if there could be any positives from Brexit, Lord Heseltine said: “You presume there are some. I do not know of a deal that is better than what we have now. And I can think of many that are a great deal worse.”

He said that in holding the referendum, David Cameron had “bowed to the pressure of the right wing of the Tory party.

“I lived through all that with John Major. They made his life hell and they’re still at it now.”

It has been widely reported that Britain’s best hope for a favourable deal lies with how much influence Angela Merkel can wield on the UK’s behalf.

Lord Heseltine said: “It will be up to Ms Merkel’s influence? For people of my generation, you can’t help but reflect that the Germans lost the war, but they won the peace.”

He also suggested the problem of mass migration might be solved by a 21st century Marshall Plan that would “persuade people to stay in the countries where they were brought up, rather than crash the borders”.

“What is happening is this thing,” he said, holding up his mobile phone. “We’ve all got them but they’ve also got them in sub-Saharan Africa. They didn’t used to see our life, day by day, in every form of media.

“Now you come to a position where the youngest, most energetic people from the less prosperous parts of the world want to come here and share in it? How do you deal with it? You have to have immigration controls but can you make them work?

“I increasingly wonder about Marshall Aid at the end of the Second World War. I read about Donald Trump cutting foreign aid, the UK cutting foreign aid, I just wonder if the Western world hasn’t got to recognise it has an obligation to rebalance the wealth it has created.