Mobile phone network 3 has been refused permission to force its rivals to disclose emails and documents in a process that would have cost each of them £200,000.
The High Court ruled that 3's requests were not specific enough.
The network is engaged in a legal battle to prove that O2, Orange, T-Mobile and Vodafone colluded to slow down improvements to the system which allows consumers to switch networks.
It wanted access to years' worth of documents and emails to boost its case, but the High Court said that its claims fell out with the rules governing pre-action disclosure.
3 claims that the four other mobile operators are behaving anti-competitively by blocking or slowing the progress of change in the rules governing subscriber switching from one network to another.
3 is a later market entrant than the others and is almost totally dependent for its business on convincing subscribers to switch to it.
The mobile number portability (MNP) system is the process by which a phone user moves their number and account from one network to another. In the UK It is a particularly cumbersome and lengthy process, taking up to a week.
The case put forward by 3 is that the four networks were keen to change and speed up the system until it entered the market in 2004, at which point they tried to block change order to protect their market share.
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