Karel De Gucht described the plans being put forward today by the Government as “very problematic” and at odds with Brussels’ ideas for a transitional period.
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 its terms.
They would be Britain abiding by decisions of the European Court of Justice, paying contributions – and not seeking to sign preferential trade deals with third countries.
However, two Cabinet ministers have already insisted that Britain must also be free to negotiate free trade agreements with non-EU countries, from day one after departure in 2019.
“It’s very problematic”, Mr De Gucht told BBC Radio 4 Today’s programme, adding: “Politically it’s very difficult because, to put it simple, we would be keeping Britain warm.”
Meanwhile, Ireland’s EU commissioner, Phil Hogan, has suggested Dublin could block a deal on customs unless it dealt with “the Irish question”.
He told the Financial Times that British ministers “still don't realise the other 27 [member states] have to agree to this transition period of two or three years or whatever they're going to be seeking”.
“There's a high level of delusion in London at the moment about what is required to be done,” Mr Hogan said.
“I'm very concerned about the Irish question. Ireland is probably the biggest victim of this mess.”
Britain has said it wants to use smart technology and spot checks to police the flow of goods between the two countries after Brexit, angering the Irish Government.
Dublin fears the impact on the peace process in the North and has pushed for the Irish Sea to become the post-Brexit border with the UK, if it insists on leaving the existing customs union.
The warnings came as David Davis, the Brexit Secretary, announced Britain will seek the same trading rules with the EU for an “interim” period, of perhaps three years.
Ministers will tell Brussels they want a “temporary customs union”, amid evidence that businesses are already pulling investment because they fear the harm to trade from a “cliff edge” in 2019.
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