Portugal: double taxation agreement, Article 29: Termination
This Convention shall remain in force until denounced by one of the Contracting States. Either Contracting State may denounce the Convention, through diplomatic channels, by giving notice of termination at least six months before the end of any calendar year after the year 1968. In such event, the Convention shall cease to have effect:
(a) In the United Kingdom:
(i) as respects income tax, surtax and capital gains tax for any year of assessment beginning on or after 6th April in the calendar year next following that in which the notice is given; (ii) as respects corporation tax for any financial year beginning on or after 1st April in the calendar year next following that in which the notice is given.
(b) In Portugal:
as respects Portuguese tax on profits, income or capital gains arising on or after 1st January in the calendar year next following that in which the notice is given.