LONDON - The border between the Republic of Ireland and Britain's Northern Ireland became a major flashpoint Sunday with British Prime Minister Theresa May urged to change her Brexit plans to prevent a mounting crisis.

Phil Hogan, Ireland's European Union (EU) commissioner warned the border issue could derail May's hopes of agreeing a trade deal with the EU after Britain leaves the bloc.

The Observer newspaper in London said May had effectively been handed just days to give stronger guarantees over the border issue.

The newspaper said the threat of a hard Irish border has emerged as a major obstacle to May's aim of securing the green light for Brexit trade talks at a crucial summit only weeks away.

Hogan, who is the EU's agriculture commissioner, told the Observer that Britain remaining inside the EU's single market and customs union, or allowing Northern Ireland to do so, would end the standoff.

However, May and her Brexit ministers have insisted that Britain, including Northern Ireland, will leave the single market and customs union in March, 2019 when it ceases to be an EU member state.

Hogan warned in his interview that there was blind faith from some British ministers that Britain would secure a comprehensive Brexit free trade deal. He warned that Ireland would continue to play tough to the end over its threat to veto trade talks until it had guarantees over the border.

The Irish government has insisted on a written guarantee that there will be no hard border between the Republic and Northern Ireland.

Britain's International Trade Secretary Liam Fox told Sky News Sunday that a final decision on the border issue can not be made until a UK-EU trade deal has been agreed.

Fox said: "We don't want there to be a hard border, but the UK is going to be leaving the customs union and the (EU) single market."

Labour's shadow chancellor John McDonnell said he was concerned by Fox's comments, warning they could put talks ahead of the crucial December summit of EU leaders in jeopardy.

Labour's shadow Brexit secretary Barry Gardiner said in a Sunday television interview his party had not ruled out Britain staying in the EU single market and agreeing a new customs union with Brussels.

To add to the wrangle, the leader of the Democratic Unionist Party Arlene Foster expressed confidence at the party's annual conference in Belfast Saturday that May's government will not agree to any post-Brexit special status for Northern Ireland.

Foster was cheered as she told delegates: "We joined the then-European Community as one nation, will leave as one United Kingdom."

Currently, May's minority government is being propped up by the 10 DUP politicians at Westminster.

The EU has given May until Dec 4 to come up with further proposals on issues including the Northern Ireland border, the Brexit divorce bill and citizens' rights. Proposals are being demanded by Brussels before European leaders agree to starting EU-UK talks on the future trading arrangements.