The Children's Investment Fund (TCI), the London-based activist hedge fund, is at the sharp end of the fight over disclosure of covert stakes in companies built up through derivatives.

TCI, run by the secretive Chris Hohn, lost a court battle against the US rail network CSX last month, with the judge ruling that the fund should have announced a holding via derivatives that peaked at almost 14 per cent - even though the Securities and Exchange Commission backed the fund's view that disclosure was not required under US law.