A former member of Margaret Thatcher's cabinet has accused the Daily Mail of "telling lies" about Ralph Miliband after the newspaper claimed that the Marxist writings of the late father of the Labour party leader meant that he hated Britain.

In the biggest blow yet to the Mail editor, Paul Dacre, who has launched a strong defence of his paper's decision to claim that Ralph Miliband had left an "evil legacy", Lord Moore of Lower Marsh said his former tutor was a good man who never had a bad word to say about Britain.

Moore, who served in Thatcher's cabinet between 1986 and 1989 and was briefly tipped as a potential successor to Thatcher, said it "beggars belief" that the Mail could impugn the patriotism of Miliband, who taught him at the London School of Economics.

Praising Miliband as a "great academic" and an inspiring teacher, Moore said: "Ralph Miliband taught me and I can say he was one of the most inspiring and objective teachers I had. Of course, we had different political opinions but he never treated me with anything less than complete courtesy and I had profound respect for his integrity."

In a statement issued to the Press Association Moore added: "He had come here as a refugee, done his duty to his adopted country by serving in our Royal Navy during the war, become a great academic and raised a good family.

"I saw him week after week and it beggars belief that the Daily Mail can accuse him of lacking patriotism. I never heard him ever say one word which was negative about Britain – our country.

"The Daily Mail is telling lies about a good man who I knew. The people of this country are good and decent too. They do not want the Daily Mail attacking the dead relatives of politicians to make political points."

The intervention by Moore came after Lord Heseltine, the former Tory deputy prime minister, accused the Mail of demeaning the political process with its attack on Ralph Miliband.

In remarks that went further than the careful response of Tory ministers, Heseltine said there was no justification for the headline on the Miliband piece which said he hated Britain.

Heseltine told The Daily Politics on BBC2: "This is carrying politics to an extent that is just demeaning, frankly. The headline isn't justified. It is completely out of context. As everybody knows the guy fought for this country and we now live in a totally different world to the clash between communism and fascism."

The former deputy prime minister addressed the Mail's claim that Marxists such as Ralph Miliband deserved to be condemned because of the repression of the Soviet Union. He said: "Let us be frank. Stalin did some of the most appalling things but the Russians turned the second world war."

Heseltine also said the Mail had published "hatchet jobs" on Ed Miliband and Nick Clegg.

William Hague had earlier said the Mail's attack had no implications for the future regulation of the press in Britain. The foreign secretary said it was understandable that the Labour leader had decided to defend his father.

Hague declined to say whether the newspaper had carried out a hatchet job as he said: "These things do happen."

Many ministers believe in private that the Mail made a serious error in saying that Ralph Miliband, who fought for Britain in the second world war after escaping the Holocaust, hated the country. But they are declining to criticise the Mail because they do not want to fuel the row, which has overshadowed the past 48 hours of the Conservative conference, ahead of negotiations over the future of press regulation.

But a member of the prime minister's No 10 policy board criticised the Mail. Margot James, MP for Stourbridge, tweeted: "Crass and cruel to condemn Ralph M'band for his Marxist views when they were formed in 40/50s, deeply misguided maybe but not unpatriotic."

Hague simply supported Ed Miliband's decision to defend his father, though he acknowledged that the Daily Mail's deputy editor, John Steafel, had said the Mail Online had been wrong to run a picture of Ralph Miliband's gravestone with the words "grave socialist".

The foreign secretary told the Today programme on BBC Radio 4: "I think it is very understandable that a son in any walk of life, not just a politician, comes to the defence of a parent. That is what we would expect to happen and that is clearly what is happening here. We should understand and respect that. I am in no position to judge myself about it and he will have known his father far better than any of us could have possibly have done."

The London mayor, Boris Johnson, told LBC 97.3 he understood Ed Miliband's reaction. "What I actually feel, I've got ancestry that doesn't come from this country and I think people do feel very sensitive, particularly if the patriotism of those relatives is impugned," he said.

"I can imagine that being a very, very hurtful thing and I would definitely want to fire back if it was me."

Charles Moore, the former editor of the Daily Telegraph, meanwhile, accused the Daily Mail of offending against taste and decency on "multiple fronts".

Moore, who is the official biographer of Thatcher, writes in this week's Spectator magazine: "The Mail managed to offend against taste and decency on multiple counts – attacking a man for his deceased father's views, misrepresenting those views, attacking a Jew, attacking a refugee from Hitler."

Downing Street is working hard to ensure that ministers keep out of the Labour row with the Mail. Ministers have been told to make a simple argument that any child would rightly want to defend a parent.

David Cameron said in his interview on the Today programme on Tuesday, which took place at 8.10am, that he had not read the Daily Mail piece. He said the same thing three hours later in separate interviews with the main television news programmes. By early afternoon, by which time he would be facing no more interviews until after the conference, Tory sources let it be known that the prime minister had read the Mail article.