College lecturers across Scotland are staging a two-day strike in a significant escalation of their ongoing pay dispute.
Members of the EIS Further Education Lecturers Association want a bigger cost-of-living pay rise.
Colleges argue many have seen large pay increases in recent years and lecturers have already taken part in four one-day strikes since January.
This is the third national industrial dispute at colleges in recent years.
Pickets will be on duty outside many college buildings but they are expected to remain open.
The EIS Further Education Lecturers Association argues the offer means a 2% consolidated rise covering a three-year period and says this is unsatisfactory.
Employers organisation Colleges Scotland says this is not the whole story.
It says the offer is a consolidated rise worth 3.5% over three years and five months - plus an unconsolidated flat cash settlement of £400.
The body also points out that many lecturers have also received substantial increases through a scheme to harmonise pay and conditions across the country.
It argues the package on offer would see nine out of 10 unpromoted lecturers earn almost £42,000 a year.
The EIS considers this to be purely a pay dispute but employers believe it also touches on conditions of service.
Talks on Monday ended without agreement but while there have been signs of progress in recent weeks, the war of words between the two sides has intensified with a series of public claims and counterclaims.
The EIS accuses Colleges Scotland of a lack of urgency in trying to resolve the dispute but Colleges Scotland insists that is not the case and claims the union's demands are "unsustainable".
Colleges Scotland also says the union is demanding a 4 day working week for lecturers although the EIS denies this and says that claim is "bizarre".
EIS General Secretary Larry Flanagan said: "There has been a distinct lack of meaningful negotiation from Colleges Scotland, and a lack of urgency on their part to resolve this dispute."
Mr Flanagan said Colleges Scotland were making "increasingly belligerent comments" about lecturing staff and said they had made a "bizarre claim" about the EIS demanding a four-day working week for lecturers.
He insisted this had "absolutely no basis in reality."
Pam Currie, EIS-FELA President added: "It is hugely disappointing that Colleges Scotland are seeking to link terms and conditions with a fair cost of living pay rise.
"We are balloting our members on national terms and conditions which we agreed with college management as a separate workstream.
"This agreement provides for flexible working for college lecturers - a practice which already exists in the sector."
Colleges Scotland says it has made a substantially improved offer to the union.
It is urging the union to call off the action and ballot its members on the overall pay and conditions package.
The strike action may lead to some assessments for students in specific circumstances being rescheduled although BBC Scotland understands the risk of disruption is very limited. SQA exams - such as Highers - will not be affected.
EIS FELA members are taking other forms of industrial action on days when they are not on strike.
This could potentially lead to delays in students getting the results of assessments or even obtaining qualifications.
However, Colleges Scotland says other college staff will be doing their best to ensure this does not happen.
The National Union of Students is calling on the Scottish government to intervene in a bid to break the deadlock.