Margaret Thatcher resuming the Tory conference hours after she was targeted in an IRA bomb attack was like Donald Trump raising his fist after his assassination attempt, her then-principal private secretary has said.

The blast – 40 years ago on October 12, 1984 – tore apart the Brighton Grand Hotel, where the then-prime minister and members of her cabinet were staying while the Conservative Party conference took place. Five people were killed in the explosion and another 34 were injured.

Mrs Thatcher and Lord Robin Butler went to Hove police station after the blast in the early hours of the morning. Lord Butler says he watched news of the injuries and deaths unfold on the television, and informed Mrs Thatcher when she emerged, smartly dressed, at 8am.

“I said to her, surely, you’re not going to continue with the party conference while some of your closest friends and colleagues have been killed and injured and some are still trying to retrieve John Wakeham. Norman Tebbit had been brought out, badly injured.

“And she said: ‘The conference is due to resume at 9.30 and it must resume on time. This is our opportunity to show that terrorism can’t defeat democracy’,” Lord Butler told the PA news agency.

In hindsight, he believes she was right.

He added: “She was right and I was wrong, and she made an important statement, really by doing that.

“It rather reminds me of – with an odd comparison – Donald Trump emerging after they tried to shoot him, and saying, ‘fight’.”

He added: “You usually don’t see politicians having to react to something very unexpected instinctively, and Margaret Thatcher’s instincts when the bomb went off were first of all to see if her husband was okay, and second, to show that she is not going to be stopped from carrying on business as usual.”

Lord Butler was the civil servant in charge of Mrs Thatcher’s Downing Street office and was helping her in the early hours of the morning as she prepared for her conference speech the following day.

He said: “So, at 2.54 in the morning, she was in an armchair in her suite. I was in another armchair 10 feet away, waiting and hoping that she wasn’t going to take too long in dealing with this document, and that’s when the bomb went off.”

He added: “I was tired and getting sleepy. And of course, the bomb woke us up – woke me up. She didn’t need any waking up. And I realised it was a bomb, it was obvious it was. I’d heard bombs before. There had been some close shaves that I’d been previously involved in.”

He recalls telling Mrs Thatcher to come away from the windows in case there was another explosion.

He said: “She said, immediately, ‘I must see if Dennis is all right in the bedroom next door.’ And so she opened the door to the bedroom and through that, you could hear falling masonry.

“It was actually her bathroom, rather than the bedroom, that was collapsing.

Mr Thatcher emerged, pulling on a pair of trousers over his pyjamas, and the three of them went out into the corridor, where they saw what looked like smoke coming from under then-foreign secretary Geoffrey Howe’s door.

Mr Howe and minister John Gummer, now Lord Gummer, emerged alongside secretaries who had been typing up Mrs Thatcher’s speech.

He said: “I’ve got this vivid memory of [Howe’s] protection officer running against the door of the bathroom of the suite trying to kick it in so he could get in. Luckily, he didn’t succeed, because the hotel had collapsed the other side of the door.”

The blast caused several floors at the front of the hotel to collapse, but the lights stayed on, Lord Butler said.

He added: “[Thatcher] was so surprised that the lights hadn’t gone out when the bomb went off. She always, after that, carried a torch in her handbag so that if there was another bomb attack that she was involved in, she would be able to find her way around.”

Lord Butler realised he needed to retrieve some clothes and documents and went back upstairs to the room.

He said: “There was quite a bit of dust, but actually, the dust hadn’t got into the wardrobe, so the clothes were all right, and we packed up the papers – not realising that the hotel was hanging by a thread above our heads.”

He said he realised later that he had “ingested a lot of dust”.

He felt more danger in his daily life after the attack. Before a trip to a conference at Dublin Castle later that year, he wrote a letter to his wife and left it at their flat in case something happened.

Patrick Magee, who planted the deadly device, was handed eight life sentences at the Old Bailey in 1986, with a recommendation he serve a minimum of 35 years.

Lord Butler sat in on the closing part of the trial.

He said: “[I] saw the winding up speech by the prosecution, and for the only time in my life, clapped eyes on McGee himself and the other members of the active service unit … my impression was there were three male members of the active service unit, two women, and how very smartly they were dressed. And you know, this was no doubt in the hope of creating a good impression on the jury.”

Mr Magee was released in 1999 under the Good Friday Agreement – having served 13 years for the crime.

Lord Butler says the Good Friday Agreement brought “a feeling of great relief and satisfaction that we were moving into a political phase and out of violence”.

The daughter of Sir Anthony Berry, the MP for Enfield Southgate who died in the bombing, has met with Mr Magee hundreds of times and told the BBC she hopes one day he will accept that any killing is wrong.

Lord Butler says: “I wouldn’t have wanted to meet him. What I feel about Patrick Magee, is that he was somebody who was devoted to his cause, and I believe that he is still devoted to the cause, not of terrorism, but of a united Ireland.”