Ministers have been warned not to try to rush through controversial surveillance powers - including accessing internet records and "bulk" data collection - amid claims they lack sufficient safeguards to protect individual privacy.
The Government is braced for a bitter parliamentary battle over its new Investigatory Powers Bill - dubbed the "snoopers' charter" by critics - which will be debated by MPs for the first time on Tuesday when it receives its Commons second reading.
The Scottish National Party has joined Labour in warning that it intends to seek major changes to the legislation, arguing that some of the measures in the Bill are of "questionable legality".
Shadow home secretary Andy Burnham has said Labour would work constructively with the Government to get the legislation on to the statute book but that "substantial changes" were needed.
The Bill marks the latest attempt to create a comprehensive legal framework covering a wide range of communications surveillance powers which the Government argues are essential to combat terrorism and organised crime.
Ministers are determined to complete its passage through Parliament by the end of the year as many of the powers in the compromise Data Retention and Investigatory Powers Act passed in the last parliament are due to expire.
The proposals have already been significantly amended after a draft Bill last year was heavily criticised by three parliamentary committees and with a number of Conservative MPs still unhappy, the Government may be forced to give further ground if it is to meet its deadline.
Mr Burnham said Labour would abstain in the second reading vote, but would be seeking amendments including the introduction of a "presumption of privacy", a clearer definition of information that can be accessed by the police and intelligence agencies, and better protection for journalists and their sources.
He said: "Britain needs a new legal framework in this crucial area that is fit for the digital age, balancing powers with proper safeguards. So Labour will put party politics aside and work constructively with the Government to that end.
"But there will be no blank cheque. The Home Secretary's Bill requires substantial changes before it will be acceptable to us. While I share her wish to see a comprehensive Bill on the statute book by the end of this year, we can't let the timetable dictate the quality."
The SNP has yet to decide how it will vote in the second reading debate, but justice spokeswoman Joanna Cherry said it was a "rushed job" and that ministers had failed to make a convincing case for the powers they were seeking.
"Many of the powers which the UK Government seeks to put on a statutory footing in the Bill are of questionable legality having regard to recent judgments of the European Court of Justice and the European Court of Human Rights and go beyond those currently authorised in other western democracies," she said.
"The SNP looks forward to working with other parliamentarians to get this right but the Government must afford sufficient time for consideration of the Bill."
The Prime Minister's official spokeswoman said ministers had already "engaged significantly" with MPs across the House regarding the provisions of the legislation.
"We are very clear on the importance of making sure that our police and intelligence agencies have the powers and the tools needed to keep us safe," she said.
"Now we need to get on with taking the Bill through the House. This is about keeping people safe."