My former employer changed my Tax Code on the last payment

I received my last payment from my former employer and noticed that they changed my tax code from 1257L to OT. I don't think HMRC has issued a notice for a new tax code and they used the 1257L in the previous payrolls. In fact, I was owed a tax refund as my YTD income is well below the tax free allowance of £12,570. Can the employer do that, change my tax code without notification from HMRC even though they used 1257L in the previous months?

Hi
Unfortunately, it is not possible to check individual tax records through this forum, if you have a question which relates to your personal tax situation or tax code please contact our helpline:  
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On this page you will also see a link to Personal tax account: sign in or set up which may be the quickest way to review information relating to pay and tax history, tax codes and how to change them.