London (CNN) British Prime Minister Theresa May has been warned that Ireland will "play tough to the end" over its threat to delay Brexit talks should no solution be found to the Northern Ireland border issue.

Throughout the negotiations, the European Union has been adamant that "sufficient progress" is required over the border issue before any talks on a future relationship between the UK and EU can take place.

May has been given a deadline of December 4 by the EU to introduce further proposals on the three main areas of contention -- the Irish border, Britain's divorce bill and the rights of EU citizens.

Britain has said it will leave both the single market and customs union when it exits the European Union in March 2019 -- a move which critics say could lead to a hard border in Ireland.

Phil Hogan, Ireland's EU commissioner, has urged May to change course and instead ensure that Northern Ireland and Ireland avoid such a scenario.

EU Commissioner of Agriculture & Rural Development Phil Hogan has warned Theresa May over Brexit.

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