Shaftesbury has asked for an extension to a deadline for rival Capco to make an offer for the West End landlord.
The two property titans have been in advanced discussions for an all-share merger, it was announced on 7 May.
The £3.5bn merger would create a sprawling real estate empire across central London destinations including the Covent Garden estate and Carnaby St.
However, an earlier deadline for Capco to announce a firm intention to make an offer, or declare it does not intend to make an offer, has been pushed back.
Shaftesbury asked for the 4 June deadline to be extended, with the takeover panel agreeing to a fresh deadline of 5pm on 17 June.
There can be no certainty that any offer will be made and a further announcement would be made in due course, Shaftesbury emphasised on Monday.