Three men have been charged in the suicide bombings that killed 52 people in London on July 7, 2005.

The trio -- Mohammed Shakil, 30, Waheed Ali, 23, and Sadeer Saleem, 26 -- are accused of conspiring with the London bombers to cause explosions on public transport and at tourist attractions in the capital.

All those charged on Thursday were from the same area of West Yorkshire, England, as three of the four bombers.

"The search is not over," head of the Metropolitan Police anti-terrorist unit Peter Clarke said, adding that he expected that there would be more arrests in the case.

The three, who were arrested last month, have been charged "that between November 1, 2004 and June 29, 2005 they unlawfully and maliciously conspired," with the London bombers and others "to cause explosions on the Transport for London system and/or tourist attractions in London," the Crown Prosecution said.

"The allegation is that they were involved in reconnaissance and planning," said Susan Hemming, of the prosecutors' office.

Bombers Mohammad Sidique Khan, 30, Shehzad Tanweer, 22, Germaine Lindsay, 19, and Hasib Hussain, 18, all died in the blasts, and the latest charges are the first to be made over the attacks.

Clarke, who is also Deputy Assistant Commissioner at Scotland Yard, said the investigation into the bombings had now reached "a new stage".

Today's charges followed 21 months of "intense investigation" by his force and West Yorkshire Police into the "murder of 52 innocent victims", Clarke said.

Clarke said he was restricted in what he could disclose about the inquiry now that legal proceedings were underway.

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

"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's probably fair to describe it as a complicated jigsaw with thousands of pieces.

"We now have enough of the pieces in the right places for us 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 attack 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."

Clarke said, "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," he said.