My parent is an Italian citizen and

Well I don’t really know the answer, but this may at least illicit further response from more knowledgeable people on the forum.
I’m not too sure you are querying just the last year’s tax or earlier years? For the latter I would have thought your mother would be correct to pay any income tax due on UK earned income in the UK, any income from Italy is another matter.
Since she has went back to Italy to live I’m guessing she would be liable from the January following her move (not too sure if this is not just 18 month after) to pay Italian tax on any income. There are more minor problems if her income is derived from UK government or some local government pensions (not state pension). At the end of the day it will be the AdE, if they query anything, who will decide if and when there was liability. I think with the UK side of things it is safe to tell them whatever the exact circumstances, see what they come back with and if any liability, then make sure they have their facts right. They invariably get their procedures right and apply them fairly, but often get their calculations wrong!

The following is of general application and may be of assistance but is not specific advice on this case.You need proper professional advice from a commercialista on the Italian Tax. Subject to that:On the basis that she lived, and made her home, in the UK from getting married and had no property/assets in Italy until 2011/2012 then up until 2011 it appears she would be UK tax resident and not Italian Tax resident. If a property was owned before then but used solely as a holiday home then that should not change the position. Anything more complicated may change that.From 2011 to 2016 it will depend on her exact circumstances and she may be resident in both countries - subject to the UK/Italy double tax agreement which only allows an individual to be tax resident in one country. If her income/investments and home were in the UK and only the Italian property was in Italy then probably UK tax resident. This is where it is dependent on the detailed circumstances. This period is also complicated by the changes which took place in the UK on how UK tax residence was to be determined (from 6 April 2013)After leaving the UK to live permanently in Italy she would cease to be UK tax resident and acquire Italian Tax residence. The date from which this would apply would be dependant on the interaction between the UK & Italian tax systems.In addition. Liability to tax can arise on income generated in a country, whether tax resident or not. Again it depends on what the income/gains are. Tax paid on such income is normally set against the tax on the same income/gains in the tax resident country (effectively one pays the higher of the 2)The 183 day rule applies to everyone, not just ex pats, but is only one of the rules. It is possible to be non tax resident in either country (because of the double tax agreement) even if you are there for more than 183 days - all depends on the circumstances. Conversely it is possible to be tax resident whilst being present for less than 183 days (for UK purposes as little as 16 days in some circumstances, and up to 5 April 2013 even 1 day)Registering as a resident for residence purposes in Italy does not mean that a person is automatically tax resident in Italy, although there is a rebuttable assumption that it does.If the income/investments are significant then professional help is strongly recommended. If not then aproaching the relevant tax authority may be OK. However, contrary to what stevewmongseaton believes, the UK authorities do not always get the procedures right or apply the rules fairly (most of the time they do but...).So far as the Italian Tax authorities are concerned I cannot commentI hope that helps to point you in the right direction.

I suspect this will be down to HMGov - as they could prove her domicile in the UK and therefore decide she was liable for tax in the UK. Being resident (assuming she was not not in the AIRE) measn she would be liable for taxes in Italy.