The workers will be on strike until Feb. 19.
Some sections of the museum will be closed and some events will be cancelled, causing an inconvenience to visitors.
"We have taken the difficult decision to cancel our program of free half-term events and activities this week to avoid last-minute disappointment for visitors. We have also taken the decision that entry to the Museum from 13–19 February will only be available to pre-booked ticket holders and Members," the museum said in a statement.
The museum offered a 4% pay rise, which was seen by the staff as "not really enough."
The UK's annual inflation stood at 10.1% in the year to January from 10.5% in December.
"It’s shaming that this historic institution is being operated on the basis of low pay for these professional & dedicated staff. Please join their picket lines Saturday and Sunday mornings this coming weekend," John McDonnell, who was shadow chancellor under Jeremy Corbyn’s leadership of the Labour Party, said on Twitter.
The PCS is one of the largest trade unions in the UK, representing more than 200,000 civil servants and government workers, including those who work at various cultural organizations such as National Museums Scotland, Historic England, the Wallace Collection, and the National Museum of Liverpool.
As a response to the protests, National Museums Scotland closed some of its venues, including the National War Museum and sections of the National Museum of Scotland, but other organizations such as Historic England, the Wallace Collection and the National Museum of Liverpool remained open.
Hartwig Fischer, director of the British Museum, previously said in a statement that he respects the decision to strike while expressing regret that family visits to the museum during the mid-term break will be affected by the strike.
The closure of some sections and services may cause an inconvenience to visitors and impact the museum's revenue.