New snooping powers forcing internet firms to store records of web and social media use have not yet been conclusively justified, a new report has found.

A committee of peers and MPs said the Home Office had further work to do before the UK Parliament could be confident that proposals to require communications service providers to collect and store data, known as internet connection records (ICRs), had been "adequately thought through".

ICRs detail services a device connects to but not users' full browsing history or the content of a communication.

The new obligation for companies to retain them for up to a year is one of the key planks of the Government's landmark Investigatory Powers Bill as ministers attempt to weigh the need to give spies and police the powers they need to track terrorists and serious criminals in the digital age against privacy concerns.

A joint committee established to scrutinise the draft legislation said it was satisfied that the value of ICRs "could outweigh the intrusiveness involved in collecting and using them".

But it warned there were "strong concerns" about the lack of clarity over what form the ICRs would take and about the cost and feasibility of creating and storing them.

It said it "has not been persuaded that enough work has been done to conclusively prove the case" for ICRs.

In a 194-page report the Joint Committee on the Draft Investigatory Powers Bill made 86 detailed recommendations.

It is the third parliamentary report to call for substantial changes in recent weeks and raises the pressure on the Conservative Government as it braces for a battle to get the proposals through Westminster.

Earlier this week, a separate committee said the bill was unclear, lacked sufficient privacy protections and failed to cover all the intrusive powers used by spy agencies.

Chairman of the committee Lord Murphy of Torfaen said: "The Prime Minister described the draft bill as being the most important in the current session.

"It is indeed significant in scale and scope and comes at a time when public debates over the tension between civil liberties and security are prominent.

"There is much to be commended in the draft bill but the Home Office has a significant amount of further work to do before Parliament can be confident that the provisions have been fully thought through."

The report also said fuller justifications were needed for bulk powers that allowed security services to collect data en masse.

It should be amended to make clear that the approach to encryption was not designed to compromise security or require the creation of "back doors" for authorities, the inquiry found.

In addition, the Government was told it needed to make it explicit that firms offering so-called "end to end" encryption on communications services would not be expected to provide decrypted copies of those communications "if it is not practicable for them to do so".