Dany Cotton is used to receiving bad news in the small hours. As commissioner of the London fire brigade, she is one of three senior officers on the brigade’s rota who will answer a call alerting them to a major incident.

Last Tuesday night it was Cotton’s turn on rota. She was six months into her job running the country’s largest fire service and had almost three decades as a firefighter under her belt. But nothing prepared her for the horrors of the ensuing hours.

“Just from hearing the initial messages, I wanted to get on the road and get there as soon as possible. I live in Kent, a fair old way away. I took the details, got a postcode, got in my car and drove on a blue light (which allowed her to drive at speed to the fire).

“When I arrived I got out of my car and looked up at Grenfell Tower. My first thought was that it looked like something out of a disaster movie, like something that could never happen in London.”

As she had made her way to west London, Cotton, 48, had been listening to her brigade radio, which was transmitting messages between the control room and the fire ground, trying to form a picture of what awaited her. When she saw the blazing tower, any hope she had that loss of life would be avoided was extinguished.

“Sometimes we have buildings that are under refurbishment, that are covered in scaffolding and netting, and when [these] catch fire it looks dramatic and everything blazes, but it doesn’t affect the flats. But I thought, ‘These are actually premises on fire,’ and it was a truly shocking moment, looking at that for the first time.”

Three months into her job as a firefighter and still a teenager, she had attended the 1988 Clapham Junction train crash in which 35 people died. “That was shocking for me. I was so new – I’d never seen anything like it. But this was shocking because I understood the severity of it very quickly. I could see while I was putting my fire gear on that there were people in there and there was going to be a loss of life.”

There was another crucial distinction. “This was a developing incident. The Clapham train crash had happened and we were dealing with an aftermath. Grenfell Tower was an ongoing and worsening situation.”

The background to the Grenfell catastrophe will become apparent in the public inquiry and Cotton is restricted from talking about the incident itself. But on the night it became clear that she and the commander on the ground had to make a vital call very early on: did the gravity of the unfolding tragedy mean that the usual protocol should be abandoned?

In extreme situations such as Grenfell Tower, the local authority will dispatch a structural surveyor. But in the early hours of the morning one was not immediately available and Cotton had to make urgent decisions about whether to commit firefighters in the absence of that formal assessment.

“I did a dynamic risk assessment and we knew we were going to be doing things that were not following our normal procedures. Had we just followed standard fire brigade procedures, we would not have been able to commit firefighters in and conduct the rescues we did.

“That’s very difficult for me. I’m in charge of London fire brigade, and I was committing firefighters into something that was very unknown and very dangerous.

“The assessment I made was that there was saveable life in that building at that time, and it was our job to go in. It was very difficult, it made me feel physically sick, looking at the building and knowing I had a hundred-plus firefighters in the building at any one time.”

The decision almost certainly averted an even greater loss of life. “There was a massive risk to my firefighters, but you balance that against the fact that you join London fire brigade to save people’s lives. We wanted to save as many people as we could. That was the bottom line.”

Cotton had gone into the tower to gain an understanding of the nightmare confronting her colleagues. “I could see the conditions when we went in, the severity of the fire. The debris was raining down on us. I knew I was committing them to an extreme situation, and in those situations things aren’t always predictable and that was very frightening.”

More than 250 firefighters were dispatched to the scene, a near-unprecedented number. “I was aware we were dispatching 40 fire engines, which is one of those things we do once in a blue moon. I’ve done it once before – for the day of the Olympic closing ceremony when a large-scale refuse fire very near a chemical hazards site, only seven miles from the Olympic park, threatened to disrupt the ceremony. Prior to that, the last 40-pump fire was about 1972.”

As she entered the tower’s downstairs lobby, Cotton was struck by her colleagues’ demeanour. “The one thing that was overwhelming for me was the calm professionalism of the firefighters; they were going into something that they knew was extremely difficult and challenging.”

Despite the frenetic pace of activity, Cotton said the fire crews were clear about their roles. “It was very structured. It wasn’t people running into the building. We have officers at different points to do checks, so we know who’s in and who’s out. When they came out they [the firefighters] were excessively tired, they were hot. They were working very hard in difficult conditions, but they wanted to go back in time and time again.

Daylight brought a new appreciation of the unfolding tragedy. “It became increasingly difficult just looking at the building and still knowing there were people in there. The pressure we all felt to keep trying and doing our best was immense.

“I personally went round to loads of my crew. I was just saying, ‘Drink more water, sit in the shade, take your fire gear off, cool yourselves down, get yourself ready to get back in.’ It’s absolutely essential for their ongoing health, to be able to deploy them again.”

It is likely that the horrors of the Grenfell Tower blaze will linger in the minds of the firefighters for years to come. “They did absolutely everything they could to get as many people as possible out, but you can’t help feeling, ‘What if?’ I know we could have done no more, and I genuinely feel if we had tried to do any more we would have risked losing the lives of my firefighters. Before they went off duty, they were all spoken to individually by a counsellor and they’ve all had follow-up phone calls. When they are back on duty next week they will have access to a counsellor, and my own in-house counselling team is being reinforced by officer support from the NHS and other fire and rescue services – there are a lot of people who need a lot of help.”

Cotton said she had been buoyed by the response of the public, who have taken to cheering fire engines passing them in the street – testimony, she says, to the courage and professionalism of her colleagues.

“In my first six months of doing this job, we’ve had two terrorist incidents and now this. And time and time again my staff step up and are immense in their response, and it just makes me so proud.”