It is a right which is recognised in international treaties, including the European Union's Social Charter. The United Nation's Covenant on Economic, Social and Cultural Rights also enshrines the right to strike. In most European countries the right to strike is enshrined in, and protected by, a written constitution. The contract of employment of those who strike is considered suspended for the duration of the strike. It is not considered to have been broken.
Exceptions are the UK and Ireland.
The UK has no written constitution and the right to strike is not protected. Dependent on which side the government of the day represents, the law is changed to favour either owners and directors or else the working population. Conservative governments, for example, in power since 1979, had by 1996 passed something like eight Acts of Parliament reducing, indeed removing, basic rights of the working population. Under this legislation any striker in the UK is considered to have broken his contract of employment as all those on strike may be sacked together without compensation. They may be sacked selectively when the strike is 'unofficial'.