employment income-income tax

Dear Sir or Madam,
Could you please clarify one case in respect of tax year 2010/2011. An employee receives his salary monthly. So from gross salary I will deduct 1/12 from Personal Allowance being GBP 539.58 and after will withhold 20% and transfer the amount of income tax to HMRC. My questions is when employer can obviously see that the employee’s income is more than 37400, should he continue charging him 20% or at the momemet he realises that income is higher the employer will start to withhold 40%? Thank you!