Further to Peter's post above, another case has been reported of a British passport holder being detained on arrival in Bulgaria and sent home next day, because born in the Irish Republic, her status was British Subject rather than British Citizen, meaning that she should have had a visa.
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This is the relevant extract from the Foreign & Commonwealth Office website:
With effect from the 1 September 2006, British citizens holding valid passports will be allowed to enter Bulgaria for up to 90 days in any 6-month period without requiring a visa, instead of the previous 30 days. Prior to travelling you should ensure that your passport has a validity of three months beyond the end of your intended stay. Damaged passports will not be accepted. You are not allowed to re-enter Bulgaria after the initial 90 days have expired if you have not spent the requisite period outside the country. (Please note that British subjects, e.g. of Irish descent and born before 1 January 1949, must obtain a visa from the Bulgarian Embassy in London prior to any visit to Bulgaria, otherwise they will be refused entry and returned to the UK.)
(Crown copyright material reproduced with the permission of the Controller of HMSO and the Queen's Printer for Scotland)
In addition to the distress, inconvenience and expense suffered in such circumstances, it is highly unlikely that any insurance policy would cover the cost of the holiday or any additional losses, as the insured person has failed to comply with official immigration/entry requirements.
Hopefully this latest unfortunate incident may serve as a reminder to others in similar circumstances to check their status and apply for a visa if necessary.
David