Those drivers are now on picket lines across the West Midlands, including Wolverhampton depot, where they have been holding banners that read: "National Express: Shame on You. Billions in the bank. Lousy pay for workers. Pay up now."
Passengers have been forced to find other ways to travel, although some have supported the walkout which is going on indefinitely.
On Monday afternoon, chiefs at National Express West Midlands apologised for the disruption.
“We are very sorry to people whose plans were disrupted by the strike on Monday," a spokesperson said.
"Our offer of a 14.3 per cent pay increase remains on the table and our door is open. With this pay increase, the average driver would earn almost £33,000 in Birmingham and the Black Country or £34,000 in Coventry.
“We want to reassure our customers we are doing all we can to resolve this issue.”
Drivers voted by 71 per cent against the latest offer in a ballot over Friday and Saturday.
Reacting to the result, Unite general secretary Sharon Graham described National Express as an "extremely wealthy company and makes considerable profits from the hard work of our members", adding: "The company must come back with an offer our members can accept."
However, bosses at the company have said its 14.3 per cent offer would have led to a starting salary of £13.49 per hour, up from £11.80 in November 2022, while those on the middle level would earn £14.74 an hour compared to £12.90 four months ago.
The highest earners would have earned £16.46 per hour, an increase of £2.06 an hour.
The company added: "This equates to almost £33,000 per year for typical full-time experienced drivers. If the offer is accepted, we will backdate to January 2023 which equates to around £900.
"(We are) also offering accident and assault pay (12 weeks average pay) and after 2pm on Christmas Eve, Boxing Day and New Year, three times the £16.46 rate."
However, union bosses have rejected the offer, saying the officer is a 'real terms pay cut'.
Tyrone Fowles, West Midlands regional leader at Unite, said: "They are getting a pay cut, in essence, from the offer that has been put forward from National Express and what needs to happen is for National Express to come back to the table and give our members a decent pay rise.
"There have been numerous negotiations with the company and different offers have been put forward, but unfortunately, they have not come back with a suitable offer and our members were balloted and that's what has led to this as they said that was not enough."
Drivers themselves have also given their reaction, saying they are stressed and want a "proper pay rise".
Speaking on the Wolverhampton picket line, one said: "We've had to do this because of the stress and the issues that we go through and it's necessary to ensure we can get a proper pay rise.
"Morale, right now, is in the garbage and you constantly go through problems every day, with people coming on with no tickets and people threatening you, so what we are being offered just isn't enough."
As the dispute rumbles on, only a much thinner service on just a handful of routes are in operation. As of Monday evening, the National Express routes still going were:
4 Birmingham - Solihull (via Sparkbrook, Tyseley, Acocks Green & Olton)
5 West Bromwich - Sutton Coldfield (via Great Barr, Kingstanding & New Oscott)
6 Birmingham - Solihull (via Sparkbrook, Sparkhill, Hall Green & Shirley)
6 Dudley - Stourbridge (via Russells Hall Hospital, Brierley Hill & Amblecote)
28 Heartlands Hospital - Great Barr (via Ward End, Castle Bromwich, Erdington, Old Oscott & Perry Beeches)
51 Birmingham - Walsall (via Great Barr & Perry Barr)
59 Wolverhampton - Ashmore Park (via New Cross Hospital & Wednesfield)
87 Birmingham - Dudley (via City Hospital, Smethwick, Oldbury & Tividale)
97 Birmingham - Chelmsley Wood (via Bordesley Green, Heartlands Hospital & Meadway)
529 Wolverhampton - Walsall (via Willenhall)
X4 Birmingham - Falcon Lodge (via Aston Expressway, Erdington, Sutton Coldfield & Good Hope Hospital)
X21| Birmingham - Bartley Green (via Birmingham University, Selly Oak & Weoley Castle)
National Express West Midlands bosses have urged customers to avoid using those routes unless absolutely necessary.
They've also said latest timetables and other information will be updated twice daily on the company's website.