The UK's Automatic Exchange of Information agreements with the Isle of Man, Guersey, Jersey and Gibraltar all give 30 September 2016 as the deadline for information on financial accounts for 2014 and 2015 to be exchanged.
Overseas territories the Cayman Islands, Bermuda, Montserrat, the Turks and Caicos Islands, the British Virgin Islands and Anguilla have also all signed intergovernmental agreements with the same deadline.
HMRC confirmed that the data began to arrive last week, and will be delivered in stages from different 'groups' of jurisdictions.
These agreements are sometimes referred to as 'UK FATCA', as they are based on the US's FATCA regime.
HMRC also launched a "worldwide disclosure facility" this month, offering the opportunity to declare offshore income and assets before the UK begins full automatic exchange of information with more than 100 other jurisdictions under the Common Reporting Standard (CRS) in 2018.
Previous disclosure facilities offered by HMRC included special terms such as penalty reductions and reduced disclosure periods in order to encourage those with tax irregularities to come forward voluntarily.
The UK's change in approach has been triggered by the impending introduction of automatic information exchange agreements between the international tax authorities. The CRS will begin between 2017 and 2018.