The City watchdog, the Financial Services Authority (FSA), has extended the deadline it set for four news groups implicated in the Interbrew leak case to hand over the leaked bid documents.

The FSA has told the Financial Times, The Times, The Guardian and Reuters that they have another two weeks to surrender the documents. The regulator wants to carry out forensic tests to find the leaker.

All four have refused to comply with the FSA's request, saying that assisting the FSA in tracking down the source of the leak would discourage other whistleblowers. The FSA has not said what further action it will take if the news groups do not comply.

The regulator of the Euronext market, on which Interbrew is listed, is also investigating whether the leak was a deliberate attempt to rig the market in Interbrew shares.