First Glasgow, which operates more than half the services in the city, has axed seven of its week-day services and cut the frequency of buses on at least 10 others, including a number of core routes into the city centre.
The changes, which are due to take effect from June 19, have not been widely publicised and appeared to take senior officials at Glasgow City Council by surprise.
They prompted anger among councillors who warned they would hinder economic recovery and leave some of the city's most vulnerable communities isolated.
Among the routes that have been axed are the 23, a 10-minute frequency service connecting Summerston, Govan, Maryhill and Silverburn shopping centre, which will now only operate at weekends, and the 16, a three-times an hour service from the city centre to Blairdardie via Anniesland.
Other routes in the east, west and south sides of the city have also been cut or reduced, though two routes have seen increased frequency and one new service will start on June 21 to coincide with the opening of the Transport Museum in the city's West End.
The latest cutbacks follow fare hikes of nearly 25% on some routes in March as the firm restructured what it said was a "complex" fare structure.
It insisted the timetable changes would have "little effect" on the majority of its passengers.
However, Strathclyde Partnership for Transport, the council-run regional transport authority, said it was concerned about the network changes and said it would have a "huge" impact on its budget for subsidising socially essential services which has already been stretched due to previous cutbacks over the past two years.
Jonathan Findlay, SPT's chairman, said the changes emphasised the need to introduce legislation to change how bus networks are managed, following deregulation of the industry 25 years ago.
He said: "I am extremely disappointed in relation to any service reductions across the bus network locally.
"This is something which has been happening for some time and is symptomatic of the de-regulated bus market.
"Effectively bus companies can do what they like provided they provide notice of any changes.
"It's unacceptable that councils throughout the SPT area are expected to pick up the tab when essential services are withdrawn.
"You can argue over what is an essential service but these are used by people to get to work, to do their shopping and to attend hospital appointments so I would argue they are essential."
The changes have been introduced ahead of tough regulations – originally due to be introduced last April but now delayed until September – which will force bus companies to maintain regular services on key routes, and introduce minimum standards on the age and quality of vehicles.
In return, Glasgow City Council has invested £30m over the past decade in improving infrastructure such as bus stops and introducing CCTV and real-time passenger information.
Alistair Watson, convener of the council's Regeneration and Economy Policy Development Committee, said the changes had damaged the partnership working between First, SPT and the council and that he would now write to First Glasgow managing director Ronnie Park, calling for an urgent meeting to discuss services he said were at "crisis point".
"We are now calling for urgent discussions between bus operators, the council and SPT.
"We have gone too far and seen cuts that have started to isolate communities, particularly those in most need. They will cause real hardship."
First Glasgow, a subsidiary of the Aberdeen transport giant First Group, saw its operating profits of £4.4 million in the 2009/10 financial year, the last for which figures are published, a drop of nearly £3.5m on 2008/9.
Along with other operators, it has faced soaring fuel prices.
In addition, passenger demand has been stifled by a slowdown on the high street which has had a knock-on effect on passenger numbers.
Alex Shearer, commercial director of First Glasgow, said: "From Sunday June 19, First will be making a number of changes to its network of services in Glasgow, virtually all of which should have little impact on most of our passengers.
"Our customers can be assured we continue to be committed to delivering the level of bus service required in Glasgow, and at a 'value-for-money' price which they can afford."
CHANGED TIMETABLE FROM JUNE 19
16 (NOW 18)
Used to go from Blairdardie via Anniesland to city centre then to East Kilbride. Now goes from the city to East Kilbride Were 103 services, now down to 97.
23 SUMMERSTON TO GOVAN VIA MARYHILL AND SILVERBURN
Used to operate at up to 10-minute frequencies.
Cancelled except for weekends.
45 GOVAN/CITY CENTRE/AUCHINAIRN
10-minute to 15-minute frequency.
32 CRAIGEND TO PARKHEAD
10 (nine return) hourly, reduced to seven hourly services.
33 PARKHEAD TO EASTERHOUSE
18 per day, half-hourly, reduced to 16 per day half-hourly, stopping an hour earlier at 4.50pm.
53 SILVERBURN TO BRAEHEAD
21 per day, half-hourly, reduced to 18 (cuts out earliest three buses).
166 CITY CENTRE TO EAST KILBRIDE
Reduced from five early morning runs (the last at 7.05) to two.
189 GOVAN TO DRUMOYNE CIRCULAR
Reduced from 40 15-minute services per day (daytime), to 20 half-hourly services.
208 SILVERTON TO BRUCEHILL CIRCULAR
30 services reduced to 29
92 PARTICK TO ANTONINE PARK VIA DRUMRY, NOW ONLY GOING TO CLYDEBANK
20 reduced to 18, changed route.
747 TOWN TO AIRPORT
Lost one service (9.30pm cancelled, latest now 9pm).
12 RUTHERGLEN TO ROBROYSTON VIA CITY CENTRE
12-minute frequency down to 15 minutes.
27A TOWN TO BISHOPBRIGGS AND KILSYTH, CHANGED TO BISHOPBRIGGS AND TORRANCE
N66 NIGHT BUS WEEKEND SERVICE
From six buses to Mountblow via Anniesland to 10 to East Kilbride via South Side.
X86 CAMPSIE GLEN AND LENNOXTOWN
Re-routed to Kilsyth and Kirkintilloch, now 10 per day.
100 GLASGOW MUSEUMS
Starts June 21.
6/17 GOVAN/ PAISLEY/SILVERBURN SHOPPING CENTRE
39 CARNTYNE/ CRANHILL/QUEENSLIE
Four hourly evening services.
243 MOTHERWELL COLLEGE TO HAMILTON (from June 10)
Two afternoon services.
323 BARRHEAD CIRCULAR
Four evening services.