The headmasters also criticised shadow chancellor Rachel Reeves after she suggested private schools faced with closures will simply have to “make efficiencies” in an interview on Sunday.

Mr Paget-Tomlinson described her comments as “somewhat dismissive”, adding: “I find it incredible. If she were to refer to almost any independent school’s Charity Commission entry, she would find that the single largest cost is staff. 

“I would be interested to sit with her and discuss how a school like mine could achieve cost savings without making members of staff unemployed or reducing the educational provision available.”

The average private school spends 70pc of its budget on staff salaries, according to the Independent Schools Council, but this is even higher at the £19,125-a-year school in rural Dorset where it accounts for more than 75pc. 

Mr Paget-Tomlinson said: “We serve a rural community, our parents are mostly small business owners, farmers, professionals, nurses, social workers, they send their children to the school and work hard to do that. 

“The policy is not clear and it may not be made clear by the manifesto. It would be useful to discuss before this arbitrary approach is taken without, in my opinion, any real thought through processes and certainly no information to schools about the nature of the policy.”

Julie Robinson, the chief executive of the Independent Schools Council, said: “Schools will be doing everything they can to protect teachers’ jobs – teachers are the most valuable asset that any school has. 

“However, the majority of our schools are small, local community schools who cannot cut their running costs by a fifth nor expect parents to pay the extra 20pc that this policy would put on their fees. 

“Between two-thirds and three-quarters of any school’s budget goes on staff costs and this policy would mean many schools will have no choice but to consider all their options, especially as we still have so little detail about the policy.”

Labour was approached for comment.