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Theresa May was plunged into a new row today after No 10 silenced a Government minister who questioned the case for leaving the European Union following Whitehall studies that predict economic damage.

Justice minister Phillip Lee was served with a warning by Chief Whip Julian Smith this morning after he said there was a “serious question” as to whether Brexit could “legitimately” go ahead.

However, former minister Anna Soubry expressed outrage at the action to silence Dr Lee.

She said No 10 should instead discipline Brexit minister Steve Baker, who yesterday dismissed civil service forecasts as “always wrong” and claimed there was a plot “to undermine our exit from the European Union”.

Mrs May, who is touring China, faced questions over Brexit and her leadership from reporters in Beijing.

She told a news conference with Premier Li Keqiang that Britain would gain the ability to strike new trade deals outside the EU and insisted: “We believe that’s in the best interests of the UK.”

However, debates at Westminster were dominated by the Whitehall studies showing that every known deal for leaving the EU will make Britain worse off.

On Twitter, ex-GP Dr Lee appeared to forcefully contradict Mrs May’s claim that the studies could be ignored as they were drafts that had not been signed off by ministers.

He wrote: “The next phase of Brexit has to be all about the evidence. We can’t just dismiss this and move on. If there is evidence to the contrary, we need to see and consider that too.

“But if these figures turn out to be anywhere near right, there would be a serious question over whether a government could legitimately lead a country along a path that the evidence and rational consideration indicate would be damaging.

This shows the PM’s challenge.” The Bracknell MP added: “It’s time for evidence, not dogma, to show the way. We must act for our country’s best interests, not ideology & populism, or history will judge us harshly. Our country deserves no less.”

Dr Lee was contacted by Mr Smith and warned not to comment about drafts in such terms. “He was told it is best not to speculate about preliminary analyses on social media,” said a No 10 source.

However, Ms Soubry said: “I do hope the Chief Whip has not spoken to Minister Lee in those terms. I am assuming Minister Baker has in any event been admonished for his appalling comments at the Despatch Box.”

Another ex-minister, Stephen Hammond, also said Mr Baker should be disciplined rather than Dr Lee.

“One can only assume he spoke to Mr Baker for rubbishing a Government document that is being briefed as fact to the Cabinet,” he said. A third ex-minister, Bob Neill, added: “It’s a good idea for ministers not to try and rubbish their own Government’s documents.”

Civil service leaders were furious about Mr Baker’s unprecedented criticism of officials, which indicated deepening alarm among “hard Brexiteers” that they are losing ground.

Bob Kerslake, the former head of the civil service, told The Guardian: “Civil servants need the support of that government.” Tory MP Antoinette Sandbach said: “Some people want to rubbish experts, but some of us rather value their opinions.”

The leaked analyses suggested that growth would be down by five per cent after 15 years under a comprehensive trade deal and down by eight per cent on a “hard Brexit” with World Trade Organisation rules.

Labour used an Opposition Day debate today to pass a “binding motion” ordering the Government to hand over the full studies to MPs.

Tories were told to abstain. Shadow Brexit secretary Sir Keir Starmer said MPs could not make informed decisions without the full picture. “Ministers cannot keep side-lining Parliament to hide the deep divisions within their own party,” he said.

No 10 played down the analyses, saying they were “preliminary studies which have not had ministerial sign-off.

They did not model for the bespoke deal the Government is looking to achieve.”

Dr Lee’s intervention followed a scathing speech in the Lords by George Bridges, a former minister at the Department for Exiting the EU, who said Brexit now looked like “a gangplank into thin air”.

The ConservativeHome website today said a straw poll of its readers suggested a big increase in the number of Tory supporters who want her to quit immediately.

In a month, the proportion had jumped from seven per cent to 26 per cent. Some seven in 10 readers thought the Prime Minister should go either now or before the next general election, which is scheduled for 2022.

Mrs May told reporters on the plane she was “not a quitter” when asked if she would fight the next general election as party leader.

Asked by the Standard about Tory fears of damaging losses in the London borough elections in May, she said: “We will fight these elections, we will fight them strong and hard.”

She appeared to brush off rumours that enough letters calling for a confidence vote in her leadership could soon be handed to Sir Graham Brady, the chairman of the Tory backbench 1922 Committee.

“Let’s talk about where we are now and what we are doing now,” she said.

“Here I am, taking a trade delegation to China, here I am going to China to enhance the British economy and enhance global Britain.”

On the first day of her trip to China, Mrs May visited Wuhan University to meet students at a UK-China Spirit of Youth festival.

She hailed educational links that would allow the two countries “to develop the bond of friendship on which this golden era of UK-Chinese relations is built”.