Builders merchant Travis Perkins’ £558m purchase of rival plumbers merchant BSS Group is still being investigated by the Office of Fair Trading.
The OFT said it needed more time to examine the deal.
The decision on whether to let the take-over go-ahead is now likely to be taken on October 19, just three days before Travis Perkins’ initially planned to complete the deal.
The deal would create the UK’s biggest builders and plumbers merchant group.
A new enlarged group might have to sell around 10 branches, out of a combined total of about 1,700, to ensure competition in certain areas.