The Director of Prosecutions has warned Nick Clegg not to block new snooping powers, it has been reported.
Keir Starmer said there was a "real risk" that terrorists could avoid prosecution if the proposed internet monitoring powers are abandoned, The Sun said.
The leaked letter was dated before Clegg spoke out against the plans, warning the new law would "not happen" with the Lib Dems in government.
And it comes after an exclusive poll for The Huffington Post UK revealed a narrow majority of the public were in favour of the plans.
Pressure for the powers to be introduced re-intensified in the wake of the killing of soldier Lee Rigby in Woolwich.
Foreign Secretary William Hague told MPs on Monday there remained a "very strong case" to extend monitoring of online activity as he was quizzed over reported UK intelligence use of a secret US internet spy programme.
Clegg blocked the inclusion of the Communications Data Bill - dubbed a "snoopers' charter" by critics - from the Queen's Speech, claiming it was "disproportionate" and "unworkable".
In a letter to Clegg seen by The Sun newspaper and dated April 23, before the Queen's Speech, Starmer warned technological advances "are reducing the communications data we have had available under the current legislative framework.
"Investigators and prosecutors need to maintain the capability that we have, and if there is a reduction, there is a risk it may jeopardise future prosecutions," the DPP told him.
"Communications data is used to support prosecutions for a range of criminal offences. But for cases such as counter terrorism, organised crime and large scale fraud, I would go so far as to say that communications data is so important that any reduction in capability would create a real risk to future prosecutions.
"It is also important evidence in cases such as offences such as stalking and harassment which can cause great distress to the individual victim. I will be sending you case studies as examples in due course.
"Of course there needs to be strong safeguards in place to ensure that communications data is not misused, and the existing legislative framework provides a good basis for proceeding and ensuring there is public confidence in how the data is utilised."