The British Prime Minister's being accused of breaking her promise - to ensure Northern Ireland is not divided from the rest of the UK after Brexit.
The prime minister has been pushing to agree a UK-wide backstop arrangement, which would see the whole UK remain in an effective customs union with the EU, after rejecting Brussels' proposal of a Northern Ireland-only solution.
However, in the leaked letter, it is revealed Mrs May admitted to the DUP that the EU is still proposing a Northern Ireland-only backstop arrangement alongside a UK-wide solution.
It would involve the North staying in the customs union and single market if negotiations collapse.
Arlene Foster says the reports "raise alarm bells".