The new rules cover financial instruments in the same company, which give a legal right to acquire shares or have a similar economic effect to shares.
Shares and such financial instruments will have to be aggregated and disclosed once over the 3 per cent threshold.
The FSA says this will ensure that they are not used covertly to influence corporate governance and/or build up stakes in companies.
An exemption has also been put in place for CfD writers acting in a client-serving capacity, to prevent unnecessary disclosures to the market.
FSA director of markets Alexander Justham says: “This is a very significant step in improving market transparency and we have brought the implementation date forward to reflect that. The new rules will resolve some of the concerns raised about the risks of market players devising ways to avoid disclosure or over-disclosing.”