In an expansive briefing, Scotland Yard's counter-terrorism chief Peter Clarke described how detectives had followed thousands of leads. He pleaded for more information from the public, and promised there would be more arrests.

Here is the full text of his statement:

The investigation into the terrorist attacks in London on July 7 2005 has reached a new stage, but it is far from complete.

Mr Clarke has been investigating the bombings since 2005

Today we have reached the stage where the Crown Prosecution Service have authorised charges against three individuals who have been in custody since 22 March.

We have reached this stage after some 21 months of intense investigation by the Metropolitan Police and West Yorkshire Police.

Following the murder of 52 innocent victims, we launched an investigation that was designed to uncover every possible piece of detail, every clue and lead, however minute.

Our aim was quite simple - to find out not only who was responsible for setting off the bombs, but also who else was involved.

As I said in July 2005, we needed to find out who else knew what was going to happen on 7 July.

I only wish that I could share with you the extent of what we have discovered - but I cannot

Who encouraged the bombers? Who supported them? Who helped them?

In looking for the answers to those questions we have gone where the evidence has taken us, following every lead until it is exhausted.

It is impossible in a few words to convey the scale of the task we set ourselves.

But if I tell you that we have taken more than 15,000 statements and followed some 19,000 leads, that might give you some idea.

'Complicated jigsaw'

Over 21 months the relentless search for every tiny detail gave us a wealth of information about the background to the attacks in July 2005.

When two of the men who have been charged today tried to leave the country on 22 March, our judgment was that the time was right to arrest them.

There are other people who have knowledge of what lay behind the attacks in July 2005 - knowledge that they have not shared with us

I appreciate that bringing these charges will have an impact on many people.

For some it will bring back awful memories of that terrible day.

For others there may be some relief that after such a length of time there is some visible progress in an investigation that, I hope for obvious reasons, has had to be conducted in secret.

We will of course share as much information as we can with the bereaved, the 966 people who suffered injuries, and the wider public.

I understand the need for people to have as much information as possible.

Of course, now that legal proceedings are under way, I am strictly limited as to what I can say.

The public will no doubt find this frustrating, as do I.

I only wish that I could share with you the extent of what we have discovered - but I cannot.

That must wait for the trial of those who have been charged, or any others who may be charged in the future.

So the detail of the evidence must wait, but it is probably fair to describe it as a complicated jigsaw with thousands of pieces.

Search 'not over'

We now have enough of the pieces in the right place for us to be able to see the picture, but it is far from complete.

Because of that, the search is not over. I firmly believe that there are other people who have knowledge of what lay behind the attacks in July 2005 - knowledge that they have not shared with us.

In fact, I don't only believe it. I know it for a fact. For that reason the investigation continues.

It is highly likely that in due course there will be further arrests

I have a simple appeal to make today. It is for those people who have information and who have not yet spoken to us, mainly in the West Yorkshire area, to come forward.

I do understand that some of you will have real concerns about the consequences of telling us what you know.

I also know that some of you have been actively dissuaded from speaking to us.

Surely this must stop. The victims of the attacks, and those who will become victims of terrorism in the future deserve your co-operation and support.

Information plea

We still need information about the four men who set the bombs off - Mohammad Sidique Khan, Shehzad Tanweer, Hasib Hussain and Germaine Lindsay.

There are still some gaps in our knowledge of what they were doing in the weeks and months leading up to the attacks.

The same goes for the three men who have been charged today.

I shall remind you of who they are:

• Mohammed Shakil, aged 30 from Beeston, West Yorkshire.
• Sadeer Saleem, age 26 from Beeston, West Yorkshire.
• Waheed Ali, age 23, recently living in Tower Hamlets, east London, but for most of his life has lived in Beeston, West Yorkshire. Until recently Ali was known as Shipon Ullah.

We need to know more about their movements, meetings and travel.

Who did they meet? Where did they go?

But as well as this, who else knew about what was happening?

We will find out, it is only a matter of time. It is highly likely that in due course there will be further arrests.

If you have any information, now is the time to give it.

You can either speak directly to your local police, or you can ring the confidential Anti-Terrorist Hotline.

The number is 0800 789321.

I am sorry that I can't take questions this afternoon. As I said earlier, I would like to give you much more detail, but I cannot.

As soon as I can give more information, I shall.