Having looked into this issue, it is the case that a Tier 2 sponsor is required to report, to the Home Office, any migrant they sponsor who is absent from work for ten or more consecutive working days without permission. This should be done within ten working days of the tenth day of unauthorised absence.
As I understand it, the Home Office has also said that if a Tier 2 migrant is absent from work without pay for 4 weeks or more in a calendar year, their leave may be curtailed. Paragraph 323A of the Immigration Rules defines the circumstances where a Tier 2 migrant’s leave may be curtailed. There are exceptions to this, including maternity or paternity leave. These rules do not prevent Tier 2 migrants from taking lawful strike action.
If the Home Office is notified of more than 4 weeks unpaid absence by a sponsor in a year, consideration will be given to the migrants leave being curtailed under the Immigration Rules. It is welcome that the Home Office will always consider the individual circumstances before deciding whether to take curtailment action.