HMRC announced in their recent Agent Update 62 that from 1 November 2017, HMRC will reject all returns with accounts or tax computations that are not in iXBRL format.
‘HMRC will now reject all returns with accounts or tax computations that are not in inline Extensible Business Reporting Language (iXBRL) format unless the directions under SI 2003/282 allow otherwise or a dispensation by HMRC has been granted. We will ask you to re-file them in iXBRL, and if a return is not re-filed by the normal filing date, a late filing penalty may be considered.Company Tax returns: format for accounts forming part of an online return
To help us process your returns quickly please do not include a covering letter or duplicate information in PDF that is already in your iXBRL documents. However it is important that where you have additional information you need to send us, for example a loss carry-back claim, you continue to send these to us as a PDF attachment.’
More information is available on GOV.UK at Corporation Tax online filing and electronic payment.