Police will be limited in their ability to protect the public and fight crime because of restrictions on the use of a key new power in the Government's draft snooping Bill, senior officers have warned.
They raised concerns that law enforcement bodies would not be able to obtain some data relating to web activity which firms will be legally required to store for first time under the Investigatory Powers Bill.
Keith Bristow, head of the National Crime Agency, said proposed rules around internet connection records (ICRs) appear to preclude access to details such as travel bookings that could be valuable in missing people inquiries. Another officer suggested human trafficking investigations could be affected.
The Home Office said it believes the access to data proposed in the Bill "strikes an appropriate balance between the operational need and the sensitivity of the data".
ICRs - which providers will have to store for up to 12 months - will detail services a device connects to but officials insist they will not cover users' full browsing history or the content of a communication.
Mr Bristow said the draft Bill "goes a long way to meeting our operational requirements".
He told a committee of MPs and peers appointed to examine the Bill: "There is one particular concern that I wanted to draw your attention to.
"We believe we need access to all of the data that is retained around internet connection records but as the Bill is currently drawn in draft form that would be limited to three purposes only, which means data would be retained by communications service providers that we could not request.
"This needs to be balanced against other requirements as well but I think it is important to recognise that does limit some of our ability to protect the public and to fight crime."
The draft Bill sets out three purposes for obtaining ICRs. They are in order to identify:
:: Who "sent" an online communication when the service used is already known, often through a process known as internet protocol (IP) resolution.
:: Which communication services a person has used, for example whether they are communicating through phone apps.
:: When a person has accessed illegal content such as a site hosting child abuse images.
The committee heard there are other reasons police may want to access ICRs.
Appearing on Monday, Mr Bristow said: "It wouldn't for instance include a website where someone could book a rail ticket, which could be hugely important if it related to a missing person.
"If we were concerned about what arrangements they may have put in place to travel we could not request access to an internet connection record to give us the lead to pursue that point.
"We are talking about websites that are not about illegal content, are not communications websites - bearing in mind these terms are yet to be defined - and thirdly IP resolution.
"If it was another type of website that would give us an investigative lead, such as one for booking travel tickets or banking, our understanding is we could not request the internet connection record of a site like that."
Richard Berry, of the National Police Chiefs' Council, offered to provide the committee with case studies "outlining some of those gaps", citing a human trafficking investigation involving flight bookings.
Asked if he was suggesting the Bill does not go far enough, Mr Bristow said: "The question as law enforcement professionals that we are seeking to answer is 'what is it that we need to protect the public?'
"But we absolutely accept there are wider considerations."
A Home Office spokeswoman said: "This Bill seeks to support the police and security services in their difficult and demanding job, ensuring they are able to keep pace with - and meet the challenges of - the modern world.
"The Government has carefully considered law enforcement's requirements and we believe the access to data proposed in the Bill strikes an appropriate balance between the operational need and the sensitivity of the data.
"It is important that this landmark legislation undergoes proper Parliamentary scrutiny, which is why these evidence sessions are a vital part of the pre-legislative process."