"It is very hard to see how being in a customs union is compatible with having an independent trade policy because we would therefore be dependent on what the EU negotiated in terms of its trading policies and we'd be following behind that", he said.
"There is more trade that we can do even before we get to those free trade agreements".
With the decision to leave the European Union, the British government regards China as one of its most important economic partners in the post-Brexit world, noted Mr Rajiv Biswas, Asia-Pacific chief economist at IHS Markit.
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Speaking in Shanghai, Mrs May suggested it would not be necessary to choose between the two, telling the: "I don't believe that those are the alternatives".
Keeping the customs union would go some way to resolving the thorny question of the Irish border, and would allow the U.K.to keep its red line of controlling immigration.
Diplomatic sources say while China has expressed concern about Brexit, it has also been reassured by messages that Britain's door is wide open to Chinese investment - a marked contrast to the suspicious reception Chinese companies have recently received in Germany.
The visit is predicted to see the United Kingdom and China sign business deals worth £9billion as Britain prepares to leave the European Union in March next year.
Documents distributed by the executive European Commission to EU national envoys, which have been seen by Reuters, set out how Brussels may try to insert terms into the EU-UK free trade agreement which London wants to start negotiating in the coming months to bind Britain into maintaining a "level playing field". "She's Britain's Iron Lady", said the Hubei Daily, an official regional newspaper, referring to the nickname of former British Prime Minister Margaret Thatcher.
He told The Sun that senior Brexiteers should stop attacking the Chancellor after he said there should only be "modest changes" to Britain's relationship with the European Union post Brexit.