BRUSSELS (Reuters) - The European Union can offer Britain more assurances over the Irish border ‘backstop’ in a political declaration on post-Brexit ties if London moves towards accepting a permanent customs union, a leading EU lawmaker dealing with Brexit said.

FILE PHOTO: Danuta Hubner, chair of the constitutional committee of the European Parliament gestures during an interview in Brussels, Belgium April 24, 2017. Reuters/Eric Vidal/File Photo

Danuta Hubner spoke to Reuters as British Prime Minister Theresa May headed back to Brussels to demand the renegotiation of the legal withdrawal treaty she agreed with the EU but cannot get ratified at home.

May wants to replace the emergency Irish border fix with unspecified “alternative arrangements” to ensure no return of extensive border checks on the island of Ireland, something the EU says is vague and not enough of a guarantee.

“‘Alternative arrangements’...relates to the future. What can be added or changed in the political declaration is to ensure some new arrangements that would solve the issue of a hard border,” Hubner said on Wednesday evening.

“Key to this is the conversation with Corbyn. Cross-party dialogue must yield some new elements for the future...The only thing we have not yet tried is a shift of the UK’s red lines,” she said of May’s talks with opposition leader Jeremy Corbyn.

Corbyn wants Britain to be in a permanent customs union with the EU after Brexit, something May has opposed so far as it would hinder Britain’s ability to pursue independent trade deals around the world.

“If there is no openness on the UK side to include those assurances in the political declaration on the future EU-UK ties, the process could mechanically take us to no-deal.”

The political declaration is a non-legally binding document that accompanies the legally binding withdrawal deal negotiated by the EU and May’s government. The British parliament resoundingly rejected that deal two weeks ago and has told May to reopen negotiations with the EU on it.

Hubner said the EU would not blink on refusing to renegotiate the withdrawal agreement or replacing the backstop.

“A step further would risk undermining the single market. If the single market loses its integrity, it would be the end of the EU,” she said. “It’s not just about Ireland.”

“The risk of no-deal Brexit has grown.”