But Mr Corbett said he would not allow firms to move in and "cherry pick" business from busy areas because he did not want the Post Office's national delivery service to be hit.
A new law comes into effect, allowing a new Post Office regulator - known as "PostComm" - to licence other companies to carry mail.
The chairman of PostComm, Graham Corbett, said he would like to see new competitors such as bus operators, which have the capacity to deliver in localised areas.
But Mr Corbett said he would not allow firms to move in and "cherry pick" business from busy areas because he did not want the Post Office's national delivery service to be hit.
"Monday will be a very important stage in the process of creating a framework for a future characterised by a healthy and competitive postal service," he said.
The move coincides with the Post Office changing its name to Consignia. The new name is designed to show that it does a lot more than deliver mail.
The Communication Workers' Union has attacked the £2m spent on the new name.
Deputy general secretary John Keggie said: "The union will work with the regulator so long as PostComm does not become the voice of big business.
'Nonsense'
"I hope the regulator will tell the Post Office to drop all this Consignia nonsense. The Post Office is the best in the world and it seems stupid to disguise that fact."
The Post Office will also become a plc on Monday, which was described by Chief Executive John Roberts as a "major opportunity" for the business.
"Our international capacity is growing fast and we want to be able to exploit it fully in the competitive global marketplace," he said.
Despite all the changes, customers will notice little difference because delivery workers will still work for the Royal Mail and high street and village outlets will still be called post offices.