The EU reassured British Prime Minister Theresa May that an Irish backstop plan would only be used temporarily, if at all, in response to a letter from May ahead of a House of Commons vote on her Brexit deal.

The U.K. and Ireland are currently part of the EU single market and customs union, with the controversial backstop ensuring an open border on the island of Ireland if an alternative agreement is not reached by December 2020.

In a letter, Jean-Claude Juncker, president of the European Commission, and Donald Tusk, president of the European Council, pledged to start trade talks as soon as possible in case the U.K. Parliament backs the draft Brexit deal Jan. 15, and to apply it provisionally pending national ratifications from the 27 EU member states.

The Parliament is likely to vote down the Brexit deal, in which case, May will have three days to present a new deal to the Parliament.

In case the proposed deal is rejected, the U.K. government could face a vote of no confidence from the opposition Labour Party, according to notes from CMC Markets UK and ING.

May previously delayed the parliamentary vote amid "widespread and deep concern" over the Irish backstop solution.