Australian bus network Kinetic and Spanish transport firm Globalvia agreed to take over Go-Ahead in June for around £647 million.
But the consortium of investors raised their takeover bid by £22 million to about £669 million on Thursday despite not having any competing suitors.
The deal will pay shareholders £15.50 for each share they own in the company, a 50p increase from the previous offer.
The offer includes a £1 special dividend per share.
In July, the bidders said they do not expect to reduce Go-Ahead’s headcount when they take over the business.
The consortium said there will be a “limited” impact on the staff employed to support Go-Ahead’s listing on the London Stock Exchange.
Australian bidder Kelsian dropped out of the race to buy Go-Ahead after falling share prices in Australia forced the group to withdraw its offer.
Go-Ahead increased profit guidance for its regional bus division last month after seeing a surge in passengers on public transport and anticipating a busy summer, particularly in tourist areas.
Michael Sewards, co-chief executive of Kinetic, and Javier Perez Fortea, chief executive of Globalvia, said: “This transaction will create a leading global, multi-modal, mass transit platform and unlock value for all stakeholders.
“Given our track record and experience we will provide long term capital and expertise to support the acceleration of Go-Ahead’s strategy and transition to net zero.”
The deal is expected to complete in October if it goes ahead as planned.