Tonga's acting Solicitor General, Sione Sisifa, says civil servants who strike on Monday will be breaking the law unless they have been granted leave by their employer.
Mr Sisifa says workers must have informed their ministry of their decision to strike and have leave approved in order to join the industrial action.
The Public Service Association wants up to 22 percent in cost of living adjustments to be paid to workers over the next two years, with an immediate commitment to at least six percent.
Its secretary general, Mele Amanaki, says over 600 members have agreed to strike on Monday unless the Government meets the PSA's terms.
The Prime Minister Lord Tu'ivakano says some people who are pushing for the strike are politically motivated because it is an election year.
He says Tonga cannot afford the demand for a 22 percent increase in wages as it is repaying a loan of up to 13 million pa'anga to China.