At the moment of the explosion there was a yellow flash. At the same time there was a tremendous pressure wave, Jacqui Putnam recalls.

"The windows imploded so the air was full of tiny shards of glass, and for the split second of the flash they were glittering.

"And then there was darkness, and this choking black smoke that was like soup; so you couldn't see anything, and you couldn't breathe. And you didn't dare move because you didn't know where the danger was, because of the holes exposing the machinery in the floor.

"Then there were screams coming from the carriage where the bomb's gone off. At the time we didn't know it was the bomb, but there were screams, the train had lost power and rolled to a halt, and then there was silence."

Putnam, a 55-year-old analyst programmer for an insurance company from Huntingdon in Cambridgeshire, was on the tube train that exploded near Edgware Road station when London was struck by a series of terrorist attacks on July 7 last year. She is still able to evoke the events of that day with almost cinematographic clarity.

Describing the dreadful journey she and her fellow survivors had to make along the length of the train towards the station, she says:

"We had to climb over debris, the train doors, and while people were waiting to climb over stuff, the rest of us came to a halt outside the carriage that had had the bomb in it. The woman in front of me looked in and started to cry, and I looked where she was looking, and then I started to cry.

"We had to climb over more stuff, and then there was Danny. Danny's legs had been taken off by the doors at the moment of the blast. He'd been knocked out of the train - and while the train was still rolling he was ricocheting between the train and the tunnel wall, and ended up half under the train. I thought that his legs were crushed by the train, which was why I couldn't see them. There was so much blood and bone I just couldn't make sense of what I was seeing. I thought, there's a wheel where his leg ought to be. I discovered later that one of his legs was up on the cables, because they'd been taken off by the doors that were blown up with him."

Putnam's experiences that day marked a psychological watershed for her. The things she saw and felt left her feeling dissociated from the world around her, and everything that had gone before.

Immediately after leaving the station, she recalls: "The effect of the blast had disoriented me so much that I didn't know how to use my phone. I looked at it, and knew I should be able to use it, but I couldn't make head nor tail of the buttons. When they put me in the ambulance to go to hospital, I couldn't put the seatbelt on, because none of it made any sense to me."

Later on, as she tried to get back to her normal life, she felt that when she spoke to people, "they were a long way away from me. I felt like I was talking to them through a barrier - like a sheet of glass - and the only people that were on the same side of the glass were the people who experienced what I'd experienced".

Putnam, like many of her fellow survivors, was diagnosed as suffering from post-traumatic stress disorder (PTSD).

Experts working with a specialist NHS team set up after the bombings to screen survivors for emotional and psychological problems believe that 20-30% of people directly affected by the attacks will have suffered a psychological disorder.

Following July 7, people have suffered an array of different problems, such as phobias and depression, but PTSD is the most prevalent problem.

According to Dr Peter Scragg, a chartered clinical psychologist at the NHS Trauma Response (London bombings) Screening Team based in central London, the symptoms of PTSD can be traced back to the initial moments of panic and can be suffered even by those who escaped serious physical injury.

He explains: "First of all the victims may have been trapped for quite long periods of time, and they would have seen some very serious injuries. Of course, what will have gone through their mind, for some of them anyway, is 'that could have happened to me', and that can lay down - particularly in that aroused state - some very scary ideas about the world.

"We see a lot of people whose key thoughts, for example, when they had this blast of very hot air, was 'I'm going to burn, I'm going to burn to death'."

PTSD sufferers can be afflicted by a variety of different symptoms. They may experience nightmares and flashbacks in which they are mentally transported back to the event. Flashbacks can be so powerful that the patient may forget where they really are. They may suffer avoidance symptoms such that they cannot bear to discuss, see or read anything that reminds them of the event. They may experience persistent arousal symptoms, leaving them unable to sleep, or may become hyper-vigilant, convinced that "there's danger around every corner". Other patients have suffered numbness - which Scragg describes as "like the brain has shut down on them" - and which can return periodically, alternating with some of the other symptoms.

These problems can have a catastrophic impact on people's lives long after the initial trauma has passed. PTSD can put considerable strain on people's relationships because of the symptoms such as irritability, hyper-vigilance and avoidance, while problems such as a lack of sleep, loss of concentration and reluctance to travel on public transport can take their toll on a career. These problems can be exacerbated if partners, families and employers fail to show the necessary levels of understanding.

Jan Pless, a marketing consultant from Melbourne, was in the UK on a work trip when she was caught in the Edgware Road bombing. She ended up leaving her job after her problems coping with the traumatic memories, coupled with an apparent lack of sympathy from her employer, proved too much to bear.

She describes her experience of PSTD as "singly the most horrifying nine months of my life. It's like being in a dark hole and you don't know how to climb out of it".

After arriving back in Australia she went straight back to work, but, after a few weeks of "doing really silly things, saying silly things" and not remembering any of it, her nine-year-old daughter persuaded her to see a doctor, "'because something's not right with your brain'".

She says her employer was preoccupied with the issue of who would cover her medical costs, and that company bosses treated her "like I had leprosy".

She had problems dealing with her work, because she found difficulties coping with uncertainty or "grey areas", and found it difficult relating to people and to everyday problems.

She says: "I didn't want to be around people. I found at work people would come to me with problems like quality control problems, and I would just look at them and think in the whole picture of things this doesn't mean anything."

Since being diagnosed with PTSD, Pless has been seeing a psychologist every two weeks, but is trying to scale down her visits to once every three weeks. She still suffers from difficulties such as inability to sleep and a fear of enclosed spaces.

Putnam is also seeing a psychologist, and feels she is now "getting back to where I was", but believes that the effects of the bombings are, to some extent, permanent.

She says: "The person who stepped on the train at Kings Cross is not the same person who climbed out of the wreckage at Edgware Road, and that's true of all of us. I'm never going to be that person again. You have to build new frames of reference around you. It takes time and you have to work hard at it."

Both women say they have benefited from therapies such as cognitive behavioural therapy and EMDR, but there may be other people with similar problems, or worse, who have gone without treatment.

The NHS trauma screening team estimates that 4,000 people may have been affected by the July 7 bombings, of whom less than 1,000 have been screened.

Scragg says: "I would imagine there are people out there who need our help. I think there will be a group of people who are frightened to come forward. It could even be that it is the people with the worst problems who are the most frightened to come forward."

· The NHS Trauma Response (London bombings) Screening Team would like to hear from you if you were directly affected by the London bombings and have not previously been assessed for treatment, particularly if you are finding your memories distressing or your sleep disturbed. You can contact them by telephone on 020 7530 3687 or via email at NHStraumaresponse@candi.nhs.uk.

Information, emotional support and practical advice is also available from 7th July Assistance.

• This article was amended on 6 September 2010. Personal details were changed at the individual's request.