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© Yves Herman / Reuters
Brexit negotiator Guy Verhofstadt has mocked Britain's plan to temporarily remain as part of the customs union and seek trade deals with other nations after it leaves the bloc as a "fantasy."

The government unveiled plans on Tuesday to continue with the current customs arrangements for at least three years after Britain officially leaves the EU in 2019. It confirms the UK will ultimately leave the system but proposes a "temporary customs union" to ensure a "smooth and orderly" transfer.

Brexit secretary David Davis has also insisted the government would "negotiate and sign" agreements with other countries once the EU withdrawal is complete, whatever the nature of any transitional period after 2019.

Verhofstadt said no deal could be struck on the UK's future trading relationship with the bloc until agreement had been reached on Britain's divorce bill and citizens' rights.

"To be in and out of the Customs Union & 'invisible borders' is a fantasy. First need to secure citizens rights & a financial settlement," Verhofstadt said on Twitter.

"Terrorists & criminals don't respect borders! Cross border instruments mean we are #SaferTogether," he added.

His remarks will be a bitter blow for Davis, who has insisted that his plans would be good for both Britain and the EU, and avoid a "cliff edge" for businesses when Britain leaves the bloc in March 2019.

Scottish First Minister Nicola Sturgeon also dismissed the UK government's plan for a post-Brexit customs deal with the EU as a "daft 'have cake and eat it' approach."

In a tweet, she said the UK should "commit to staying in the single market and customs union, period."

Former EU trade commissioner Karel De Gucht has warned that Brussels will find Britain's proposals "very problematic."

The Belgian, who was European commissioner for trade between 2010 and 2014, suggested the EU would accept the status quo on customs for several years only on its terms. They would be Britain abiding by decisions of the European Court of Justice, paying contributions, and not seeking to sign trade deals with third countries.

De Gucht told the BBC the EU would not concede the advantages of a "temporary benefits union" if that meant "keeping Britain warm" for preferential agreements with non-EU countries in future years.

A European Commission spokesman said they will study the UK's proposals ahead of the next negotiating round starting in two weeks.

"As [chief EU negotiator] Michel Barnier has said on several occasions, 'frictionless trade' is not possible outside the single market and customs union," the spokesman said.