They become the latest public transport operators to cut timetables in the face of rising numbers of workers forced to take leave due to the Omicron variant.
Ferry firm CalMac will introduce an “essential services” timetable from Monday until at least 23 January, while ScotRail, Scotland’s main train operator, will cut around 160 of its 2,100 daily services from Tuesday until 28 January.
The Scotsman understands several other bus firms across Scotland are facing similar difficulties, on top of a shortage of drivers caused by Brexit, higher lorry-driving salaries and driving test delays.
Lothian could not be contacted on Saturday, but industry sources said services could be reduced by 10-20 per cent.
The operator said staff self-isolation and rising sickness levels were “placing considerable pressure on our ability to deliver our services to the levels our customers would expect”.
In a statement published on Friday, it said: “To ensure the stability and reliability of our network, we have taken the decision to operate a Saturday service, with enhanced school journeys, across all Lothian and Lothian Country services from 6 January until 22 January.
“East Coast Buses will return to a normal timetable from 5 January.
“We will, however, operate the school services which would ordinarily run on weekdays on Services 11, 42, 44, 45 and 400.
"We are incredibly grateful to the teams across our business who have continued to work around the clock to deliver for our customers during these uncertain times, maintaining vital links across Edinburgh and the Lothians.
"Their patience and understanding is greatly appreciated.”
The firms reminded passengers to wear a face covering and keep bus windows open.
Edinburgh Bus Users Group, which campaigns for passengers, said: “We're sorry to hear Lothian has to cut services due to the pandemic and hope they'll be restored soon.
"Bus staff have done an outstanding job keeping services going.
"As users, we owe it to them to take sensible precautions when travelling.”