Sheikh Jassim bin Hamad al-Thani has submitted a new offer directly to Manchester United owners, reports the Daily Mail.
The fifth bid went in earlier this week, and the Sheikh told them that he will engage up until Friday.
The bid will stand after his deadline, but there will be no further negotiations.
The fresh offer is still for the total control of Man United, and it includes a significant further sum for investment in infrastructure and recruitment.
Sir Jim Ratcliffe had been tipped to emerge as the favourite in the takeover process for the Red Devils.
However, the latest high-stakes move from Sheikh Jassim could change things.
Whether it is enough to persuade the Glazers to part with total control of Man United remains to be seen.
The Qatari wants to buy the club, and the upcoming transfer window has triggered his latest bid.
There is a strong desire on his part to provide much-needed transfer funds, and getting the takeover done in time will be hugely beneficial.
Man United manager Erik ten Hag wants to get his transfers done as soon as possible, and having the new players involved in pre-season will be great.
However, uncertainty over ownership will undoubtedly affect his transfer plans, and Sheikh Jassim wants that to be sorted out as quickly as possible.
The Glazers still want £6 billion for Man United, but the improved offer from the Qatar Islamic Bank chairman does not meet that figure.
