BRUSSELS (Reuters) - European Council President Donald Tusk said on Tuesday British Prime Minister Boris Johnson’s demands to drop the Brexit backstop came with no “realistic alternatives” and amounted to seeking a return to controls along the sensitive Irish border.

FILE PHOTO: European Council President Donald Tusk attends a news conference after the European Union leaders summit, in Brussels, Belgium, July 2, 2019. REUTERS/Francois Lenoir/File Photo

Tusk was responding to a letter in which Johnson said the backstop - an insurance policy to preserve open borders on the island of Ireland after Britain leaves the European Union - must be erased for Britain to ratify its stalled EU divorce treaty.

“The backstop is an insurance to avoid a hard border on the island of Ireland unless and until an alternative is found,” Tusk said on Twitter. “Those against the backstop and not proposing realistic alternatives in fact support re-establishing a border. Even if they do not admit it.”

His comments were echoed by the EU’s executive European Commission, which said the backstop was the only way agreed so far by the bloc and Johnson’s predecessor Theresa May that would avoid the reimposition of full Irish border controls.

“The letter does not provide a legal operational solution to prevent the return of a hard border on the island of Ireland,” Commission spokeswoman Natasha Bertaud told a news briefing. “Our position on the backstop is well known...It is the only means identified so far by both parties to honour this commitment.”

The EU wants to ensure that its only land border with the United Kingdom after Brexit does not become a back door for goods to enter the EU’s single market - which guarantees free movement of goods, capital, services and labour.

Ireland, backed up by Brussels, says checks could also unravel the 1998 Good Friday agreement that ended decades of sectarian conflict between unionists who wanted Northern Ireland to remain British and Irish nationalists who want Northern Ireland to join a united Ireland ruled from Dublin.

Johnson is mounting a fresh campaign to convince the EU to drop the backstop, vehemently opposed by Brexiteers as it could tie Britain to EU customs and other trade rules for years after Brexit pending a new trade deal with the EU, limiting its ability to pursue sovereign trade pacts around the world.

The backstop was a key reason for the British parliament refusing on three occasions to ratify the withdrawal deal reached by May’s government with the EU in late 2018. May resigned in July and Johnson took office with a pledge to carry out Brexit by Oct.31 with or without a transition deal.

Johnson is due to meet German Chancellor Angela Merkel and French President Emmanuel Macron later this week for the first time as prime minister. But the EU has already told him it would not alter divorce terms.

A Finnish government spokesman said on Tuesday this was also the message Johnson heard from Prime Minister Antti Rinne during a phone call the previous day.

“Prime Minister Rinne reiterated that the withdrawal agreement will be not reopened,” the spokesman said.