Two MPs have launched a High Court challenge against Government legislation which gives police and security services access to people's phone and internet records on the grounds that it is "incompatible with EU law".
Conservative former shadow home secretary David Davis and Labour backbencher Tom Watson and other campaigners are arguing the Data Retention and Investigatory Powers Act (Dripa) does not contain sufficient safeguards to protect the public.
The Act was rushed through Parliament in three days in July last year with the backing of all three major party leaders.
Dinah Rose QC, appearing for both MPs who sat in front of her at London's High Court, said: "The claimants I represent are both distinguished Members of Parliament who are not very often to be seen sitting next to each other on the same front bench."
Ms Rose said that, as MPs, both men had particular reason to seek to protect the confidentiality of their contacts with constituents and other members of the public - including whistleblowers - who might approach them with sensitive information.
Both fully appreciated the importance of communications data in relation to the fight against crime and terrorism, and nothing she would say in court should be taken to detract from that.
In a legal challenge backed by Liberty, Ms Rose said: "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."
The QC is asking Lord Justice Bean and Mr Justice Collins to rule that Dripa is 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 hearing is expected to last two days.