High Court judges will give their decision today on an accusation that the Government has imposed laws which allow the police and security services to "spy on citizens" without proper safeguards.
Two MPs and other campaigners are seeking a ruling that the Data Retention and Investigatory Powers Act (Dripa) is "incompatible with EU law".
Conservative former shadow home secretary David Davis and Labour backbencher Tom Watson say they are concerned to protect the confidentiality of their contacts with constituents and other members of the public - including whistleblowers - who might approach them with sensitive information.
Their case is backed by the human rights pressure group Liberty.
A judgment is being given today following a hearing before Lord Justice Bean and Mr Justice Collins in London.
At the hearing, Dinah Rose QC, for the MPs, said the legislation was rushed through in July last year "with enormous speed and hardly any scrutiny".
Ms Rose said both MPs appreciated that the authorities needed powers to fight crime and terrorism.
"Their concern is that this legislation doesn't contain the necessary minimum safeguards to protect against the risk of arbitrary, disproportionate or abusive retention and use of personal data, and for that reason it breaches the fundamental right to privacy."
Richard Drabble QC appeared on behalf of two other applicants for judicial review, Peter Brice and Geoffrey Lewis, and agreed that there were "fundamental legal reasons why the regime must fall".
Both QCs have asked the judges to rule Dripa incompatible with Article 8 of the European Convention on Human Rights and with the EU Charter of Fundamental Rights relating to respect for private and family life and protection of personal data.
The application for judicial review was opposed by Home Secretary Theresa May.
A Government legal team, led by James Eadie QC, argued that provisions contained in the new data retention laws, combined with those already found in the Regulation of Investigatory Powers Act 2000 (Ripa), provide adequate safeguards and make the scheme compatible with EU law.