The European Commission is expected to formally block the London Stock Exchange-Deutsche Boerse deal before the end of this month.
The tie-up has been on the ropes since 26 February, when the London Stock Exchange indicated it was unwilling to meet a European Commission demand for it to divest its Italian trading business MTS.
Sources close to the deal are expecting the body to veto the deal, and a formal notice is expected either this week or next.
Next Wednesday, 29 March, has been noted as a potential date for a decision.
The official deadline for the European Commission to make a ruling on the £21bn merger is 3 April.
The London Stock Exchange and Deutsche Boerse agreed the terms of their deal in March 2016.
It marked the third formal attempt for the companies to merge.