Patients with appointments at Peterborough City Hospital are being warned to leave extra time to travel to the site while temporary traffic lights are in place.
There are long delays on the roads outside the hospital while works are taking place, meaning buses are not able to drop patients at the door of the hospital.
There have also been reports of ambulances being stuck in traffic jams outside the hospital.
The works are thought to be tree pruning works carried out by Peterborough City Council.
A spokesman for Peterborough City Hospital said: "Tree cutting is taking place on Bretton Gate near the entrance to the Peterborough City Hospital site. Temporary traffic lights are in place while work is carried out. It's not expected to last all day, but be aware this is causing some delays getting on to the hospital site. If you have an appointment for this afternoon, please leave extra time if possible. Thank you."
Stagecoach Buses said they had altered arrangements as a result of the delays. A spokesman said: " Due to a 4 way temporary traffic light system at the City Hospital there are serious delays to our services 2, 3 & 4's. We will not be serving the main doors of the hospital to try and alleviate some of these delays while these temp lights are up."
A spoeksperson for Peterborough City Council said the Bretton Gate traffic lights have now been changed from a timed setting to a manuel one, which is helping to improve traffic flow.