The pro-Remain London Mayor insisted the “last thing” Britain needs is “more chaos and uncertainty” during the coronavirus crisis. Mr Khan’s letter to the Chancellor of the Duchy of Lancaster comes as the fourth round of trade talks between the UK and the EU start on Tuesday.
The London Mayor said: “No one anticipated when voting for Brexit that the key negotiations would take place at a time when all governments are rightly focused on fighting a global pandemic.
“The last thing the country needs as it tries to find a way back from the devastation wreaked by coronavirus is more chaos and uncertainty.
“I urge the Government to put political ideology aside and pursue the pragmatic route of seeking an extension to the negotiations so that we and our European partners can focus on recovery from Covid-19.”
Mr Khan said coronavirus is the “biggest health, social and economic emergency” since the Second World War.
He continued: “Further disruption, in the form of tariffs, red tape and new immigration rules when we leave the single market and customs union at the end of the year, would be needless and hazardous.
“Close regulatory alignment with the EU has never been more vital to protect our economy and support its recovery from the impacts of Covid-19.”
The London Mayor added that an extension to the December 31 2020 deadline is “urgently needed”.
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Britain is in a transition period with Brussels until the end of the year as the two sides negotiate a free trade deal.
During the period the UK follows the EU’s rules but has no say.
Trade talks have been continuing remotely during the coronavirus pandemic but little progress was made in the third round of negotiations.
Key sticking points include fishing arrangements and the bloc’s demand for a level playing field on standards.
The EU’s chief negotiator Michel Barnier has said that the EU is “open” to a two-year extension.
In a letter to the Westminster leaders of the SNP, Liberal Democrats, Plaid Cymru, SDLP, Green Party and Alliance Party, Mr Barnier said the option is available if the UK wants it.
The group had written to the EU’s chief negotiator on May 15 demanding an extension to the transition period.
In his letter last Wednesday, Mr Barnier said: “Such an extension of up to one or two years can be agreed jointly by the two parties.