Police will not be able to use water cannon to control serious public disorder, Theresa May has confirmed.
The Home Secretary said she had decided not to authorise forces in England and Wales to deploy the Ziegler Wasserwerfer 9000.
The Metropolitan Police has said it is "naturally disappointed" by Theresa May's decision.
The move could pave the way for a row after London mayor Boris Johnson approved the purchase by Scotland Yard of three of the cannon second-hand from German police last year at a cost of more than £200,000.
David Cameron and Boris Johnson have laid wreaths at the 7/7 memorial in Hyde Park at the beginning of a day of remembrance ten years after terrorists detonated four bombs across the capital killing 52.
Mayor of London Boris Johnson says the 7/7 killers "failed in their aim", but warned that the problem has not "gone away."
Speaking in Hyde Park ahead of a memorial service, he said the terrorists "didn't in any way change the fundamentals of London and what makes this city great", adding: "Indeed, it's gone from strength to strength in the 10 years since.
"I think most people would say that London has become even more cosmopolitan, even more welcoming."
But Mr Johnson said people must remember that "the problem has not gone away", adding: "We're as ready as we think we can be, and as you can imagine there are preparations going on all the time in the event of all kinds of attacks."
A video has emerged appearing to show Mayor of London Boris Johnson telling a black cab driver to 'f*** off and die' during an altercation.Read the full story ›
Ministers should be allowed to campaign for the UK to exit the European Union in an in-out referendum, Boris Johnson has said.
The Mayor of London said it would be "safer and more harmonious" for David Cameron to allow ministers to campaign on the opposing side.
Mr Cameron was criticised yesterday for claiming that he had been "misinterpreted" when he appeared to suggest ministers would be forced to quit if they called for a No vote.
Asked if ministers should be allowed to campaign for "Brexit" and keep their posts, Mr Johnson told LBC's Nick Ferrari: "I don't see why not myself."
He added: "Do you really need to bind everybody in?"
Britain must be prepared to walk away from the EU if sufficient reform is not achieved, Boris Johnson has warned.
In his first speech since being returning to Parliament, the Mayor of London said he believed David Cameron would be successful in his efforts and congratulated him on his "schmoozathon" around Europe.
But Mr Johnson said Britain should be ready to "strike out" if terms were not agreed, suggesting an alternative future could be "just as glorious and just as prosperous".
"We now have a government, a Tory majority, with a clear mandate to seek change and therefore a Government in the most powerful position in our lifetimes to deliver reform and improvement in Europe," he said.
"If we don't get the deal that is either in the interests of this country or of Europe, then we should be prepared to strike out and forge an alternative future that could be just as glorious and just as prosperous with a free trading arrangement."
Mayor of London Boris Johnson, arriving for his count in Uxbridge and South Ruislip, said that if the exit poll was right, "then obviously, it's a very, very clear victory for the Conservatives and a very bad night for Labour"
Mayor of London Boris Johnson has offered the Duke and Duchess of Cambridge "huge congratulations" on the birth of their daughter.
Huge congratulations to the Duke & Duchess of Cambridge on the birth of their baby girl. I know Londoners will join me in wishing them well.
Boris Johnson has said Ed Miliband "would do more damage to this country than he did to his brother".
The Mayor of London accused Mr Miliband of "backstabbing" in an interview with the Sun On Sunday - a reference to the Labour leader's victory over his brother David in the party's leadership contest in September 2010.
In a heated exchange on the BBC's Andrew Marr Show, Ed Miliband suggested Tory election adviser Lynton Crosby "had put" Boris Johnson up to the comment, telling the Mayor of London: "Come on Boris, you're better than that - don't just do what Lynton says to you."
But Johnson hit back, saying: "I'm not saying your brother presented himself at A&E with a dagger in his back but he [Ed Miliband] would do more damage to this country than he did to his brother".
Boris Johnson has said speculation Conservative candidates and donors want him to take over from David Cameron is "nonsense".
The Sunday Times reported today that the party's donors and prospective MPs are increasingly unhappy with Mr Cameron's election campaign and want the Mayor of London to become Tory leader.
"They don't - that is nonsense," Mr Johnson told the BBC's Andrew Marr.
The mayor insisted a Tory majority was "not so very far from our grasp" on May 7.
Johnson, who is standing in Uxbridge and South Ruislip, has long been touted as a potential successor to Mr Cameron - even by the Prime Minister himself last month.