Fidelity check includes email, SMS and browsing history.
Research undertaken at Oxford University suggests that an increasing number of people are spying on their partners online.
One in five couples admitted to reading a partner's emails or text messages, and 13 per cent examined their partner's internet browsing history. More than 2,400 individuals were questioned in the research.
Six per cent of the couples had met online, of which over a third had met at an internet dating site and 19 per cent in a chat room.
Online daters choose more diverse partners, according to the research, as 36 per cent of the 'online' couples had different levels of education compared with 21 per cent of 'offline' couples.
Bill Dutton, director of the Oxford Internet Institute, said: "This study is a dramatic illustration of the potential for the internet to reconfigure social relationships."
The survey also found strong evidence to suggest that couples believe 'online' relationships to be just as important as 'offline' ones.
Around 85 per cent of respondents disapproved of a partner 'flirting' online with someone else.
This rises to 89 per cent for 'communicating relationship troubles to others', 94 per cent for 'having cyber-sex' and 97 per cent for 'falling in love', raising interesting questions about the three per cent who are fine with this.
One of the biggest areas of disagreement is on the viewing of pornography online. Some 46 per cent of couples disagreed on its acceptability, but men are apparently much more tolerant than women of this activity.