The 60-second TV spot, which breaks on Sunday nationwide, marks the first work by Mother London since winning the account in March.
The ad opens on a rural Post Office, before cutting to 'Life on Mars' actor John Henshaw, who plays sub-postmaster Ken.
Ken has called his team in early to rally the staff and boast their flagging morale ahead of beginning work.
Before addressing his team, Ken treats an infestation of ants with ant killer powder, and comments, "Right that should be the end of the them", in a reference to the Post Office's long-running family of animated ant characters.
The animated ants, which were created by Publicis, were used to sell the retailer's range of over-the-counter services and financial products, but have now been ditched in favour of a celebrity led-campaign.
Ken then embarks on a rousing speech, extolling the range of services the Post Office has to offer such as insurance and competitive currency exchange rates.
During the speech Ken says of the Post Office: "That's not just a logo and we're more than just a brand. We're a national institution."
Yet, despite the triumphant tone of the ad, it is never too far away from a self-mocking joke at the Post Office's expense, with Ken making a reference to the poor state of the carpet in his store.
The ad closes on the staff serving Joan Collins, the Hollywood star who featured in Cinzano's classic 70's TV campaign, as their first customer of the day. It ends with the Post Office's new line, "The People's Post Office".
The spot was directed by comedy actor, Armando Iannucci. Other celebrities set to appear in Post Office ads in the coming months include Bill Oddie, Westlife, Keith Harris and Orville.
Gary Hockey-Morley, marketing director at the Post Office, said: "The TV advertisements will form the backbone of a major new marketing campaign highlighting the uniqueness of our service and the broad range of competitively priced products on offer at local Post Office branches.
"I am particularly pleased that our people have been involved in the creative process from the very beginning; we have used their valuable input in helping to create a relevant and engaging campaign for our customers."
The Post Office is attempting to bat off increased public pressure and bad publicity about the Government's plans to close 2,500 branches by 2009 and is currently revamping 70% of its branches in the UK.
View the ad .