The extension of the Midland Metro tram from Snow Hill to New Street station will open in a piecemeal fashion from next month, it has been announced.
Transport body Centro said the first part of the £128 million line - from Snow Hill to a new stop in Bull Street - would be operational from December 6.
However, the entire 0.7-mile extension will not open in one go in order to minimise disruption to the Christmas trading period.
The entire project was due to be completed during 2015 but issues surrounding the construction of John Lewis and the Grand Central shopping centre have delayed completion.
Passenger services to New Street station will start once all infrastructure work and testing has been completed but Centro said unforeseen challenges meant it could not yet give an exact opening date for the rest of the line.
Centro chief executive Geoff Inskip said: "We recognise this is a vital time of year for retailers and the need to provide the most attractive shopping environment possible which is why we have scaled back our works.
"Doing so shows Birmingham is very much open for business and can lay on a truly magical festive atmosphere for everyone coming to the city.
"There has been a tremendous effort from everyone involved in this project with people working around the clock in recent weeks.
"It is because of that hard work we will be able to start running trams into the city centre at Bull Street before Christmas.
"However, as with all tram extensions through city centres at the moment, there are always unforeseen challenges and that makes it difficult to give an exact date for when we will run trams to New Street station."
Trams last ran on the streets of Birmingham city centre in 1953.
The Snow Hill to New Street extension includes a £40 million fleet of new trams and is expected to create more than 1,300 new jobs, adding £50 million a year to the regional economy.
Work has also begun on extending the route from New Street station to Centenary Square with services expected to start running in 2019.
Jonathan Cheetham, chairman of Retail Birmingham Business Improvement District, said: "We are delighted that Centro have considered the needs of businesses in Birmingham in this vital trading period.
"We are looking forward to the new year and the delivery of the tram through to New Street in yet another phase in the aspirations to achieve world class connectivity in transport throughout the city."