Brussels: European Council President Donald Tusk on Tuesday rejected UK Prime Minister Boris Johnson's proposal to scrap the "unviable" and "anti-democratic" backstop plan for the Irish border.

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

The EC President's tweet comes in response to Johnson's four-page letter to him on Monday where the British Prime Minister said the backstop risked undermining the Northern Irish peace process, the BBC reported.

The backstop, part of the withdrawal agreement negotiated by former Prime Minister Theresa May which has been rejected by Parliament three times, is an insurance policy to prevent a hard border in Northern Ireland.

If implemented, it would see Northern Ireland stay aligned to some rules of the EU single market.

Johnson described the arrangement as "inconsistent with the sovereignty of the UK" and insisted it could not form part of a withdrawal agreement.

If the plan was removed, the Prime Minister claimed that a Brexit deal would be passed by parliament.

Johnson also called for "flexible and creative solutions" and "alternative arrangements" - based on technology - to avoid a hard border.

He said the backstop should be replaced with a commitment to put in place such arrangements as far as possible before the end of the transition period - currently the end of 2020 under Mrs May's deal.

The EU has consistently insisted the backstop must remain part of the withdrawal agreement and cannot be changed, the BBC reported.

In a phone conversation with Johnson on Monday evening, Irish Taoiseach (Prime Minister) Leo Varadkar reiterated that the agreement could not be reopened and the backstop could not change.

Reacting to the letter, the opposition Labour Party pointed out that Johnson had previously voted for May's deal, including the backstop.

"Whichever Brexit outcome he pursues, whether it's a disastrous no deal or this fantasy land wish list, Boris Johnson clearly has no qualms about putting jobs, rights, prosperity or peace in Northern Ireland at risk," shadow Northern Ireland secretary Tony Lloyd said.

Johnson's letter came ahead of meetings this week with German Chancellor Angela Merkel and French President Emmanuel Macron on Wednesday and Thursday.