Proposed new snooping laws are unclear, lack sufficient privacy protections and fail to cover all the intrusive powers used by spy agencies, a parliamentary inquiry has concluded.
In a blow to the Government's hopes of securing smooth passage for the landmark Investigatory Powers Bill, the influential Intelligence and Security Committee claimed it was "handicapped from the outset" and suggested that even officials working on it were unclear about its intended goals.
It said the proposals in their current form represent a "missed opportunity" and called for ministers to make a number of substantial changes.
Tory MP Dominic Grieve, chairman of the committee, said: "The issues under consideration are undoubtedly complex, however it has been evident that even those working on the legislation have not always been clear as to what the provisions are intended to achieve.
"The draft Bill appears to have suffered from a lack of sufficient time and preparation."
The committee of MPs and peers took evidence in private from Home Secretary Theresa May, the heads of GCHQ, MI5 and MI6, and other officials.
Its report focused on the aspects of the Bill relating to intelligence agencies.
It said it was "surprising" that protection of privacy "does not feature more prominently" given the outcry that followed revelations by former CIA contractor Edward Snowden.
The committee warned that while recent terrorist attacks have highlighted the importance of the work performed by the agencies, "this cannot be used as an excuse to ignore such important underlying principles or unnecessarily override them".
Mr Grieve said the Bill adopts a "rather piecemeal" approach to privacy which lacks clarity and called for a new part to be added which is dedicated to protections.
The committee said the Bill's provisions covering three key powers - equipment interference (EI), bulk personal datasets (BPDs) and communications data - are "too broad and lack sufficient clarity".
EI is used to hack into suspects' smartphones and PCs and is seen as an increasingly important tool as advanced encryption makes monitoring targets more difficult.
Bulk personal datasets contain information about a wide range of people, most of whom are not of interest to the security services. Communications data - the who, when and where but not the content of an email or phone call - are central to most operations.
The report found:
:: There was not "sufficiently compelling" evidence of the need for bulk EI warrants, and members recommended they are removed from the Bill.
:: "Class" BPD warrants, which allow agencies to obtain any number of datasets in a category without specifically informing ministers of each one, should be removed because the acquisition of any BPD is "sufficiently intrusive that it should require a specific warrant".
:: The approach in the Bill towards examination of communications data is "inconsistent and largely incomprehensible".
It is the second time in a matter of days that the Bill has come under fire in a parliamentary report.
Last week the Commons Science and Technology Committee found key parts were confusing and warned the Bill could undermine the UK's technology sector. It has also been attacked by internet giants.
When the Bill was presented last year, Mrs May hailed it as a "modern legal framework which brings together current powers in a clear and comprehensible way".
Rachel Logan, of Amnesty UK, said: "The damning report from the Intelligence and Security Committee is just the latest in a relentless wave of criticism the Government has faced over its surveillance power-grab plans."
Renate Samson, the chief executive of Big Brother Watch, said: "The Intelligence and Security Committee have today dealt a serious body blow to the draft Investigatory Powers Bill. Once again the proposals have been defined as too broad and lacking in clarity."