The MP says he has joined Jo Swinson’s party to fight No 10’s ‘scorched earth approach’ to leaving the EU

Boris Johnson has suffered a fresh blow as the former Tory universities minister Sam Gyimah dramatically defected to the Liberal Democrats, accusing the prime minister of “veering towards populism and English nationalism”.

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In a major coup for the Lib Dem leader Jo Swinson at the start of her party’s annual conference, Gyimah said he had left the Conservatives to fight against the government’s “scorched earth approach” to delivering Brexit regardless of the cost to the country.

Announcing his defection exclusively to the Observer as his new party gathered in Bournemouth, Gyimah said centrists were being “cast out of both main parties”. The East Surrey MP called on them to unite and fight back against the drift to the extremes.

“I listen to ministers undermining the courts,” he said. “Ministers questioning experts because their views are inconvenient for what the government is saying about no deal. You have a government that says law enforcement is the centrepiece of its platform, and yet says in another breath that it will pick and choose what laws it chooses to respect.

“This is in many ways undermining key pillars of our constitution and the functioning of our democracy. The issue for me is not just Brexit. It is beyond Brexit – how you conduct politics and the veering towards populism and English nationalism.”

Gyimah is the first of the 21 Tory rebels who were stripped of the party whip for voting against the government for a bill to block no deal to join the Lib Dems. Swinson’s party hopes that support from liberal Tory voters can deliver them their best ever general election result should a poll take place in the next few months.

The former minister, regarded as a rising star in Theresa May’s administration, is the sixth MP to defect to the Lib Dems this year and follows former Labour MPs Luciana Berger and Angela Smith, and fellow former Tory MP Phillip Lee.

The latest defection will be a further boost to Lib Dem morale as the likelihood of a general election grows amid continuing Brexit turmoil. Gyimah’s switch of allegiance means the Lib Dems now have 18 MPs, up from 12 at the 2017 election. The party has recently updated its targets for a snap election and believes that there are now more than 100 seats “in play”.

Writing today in the Observer Swinson says: “The Liberal Democrats are the strongest Remain party in the UK, and we continue to grow, adding members, councillors and MPs. When a general election comes, we will be ready for it and ready to take our clear, pro-European message to the country.”

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In an attempt to further sharpen the party’s pro-remain message, conference delegates will vote on Sunday on whether to reverse Brexit without a referendum by revoking article 50 if they form the next government. Ahead of that vote Swinson says: “We want to stop Brexit, and if the Liberal Democrats win the general election then we will revoke article 50. Our country deserves better than Jeremy Corbyn and Boris Johnson. The Liberal Democrats are the positive, hopeful alternative that the country needs.”

The defection came as Johnson prepared to strike an uncompromising tone at a key Brexit meeting in Luxembourg on Monday with European commission president Jean-Claude Juncker and the EU’s chief Brexit negotiator Michel Barnier. He will tell the two EU power brokers he will refuse to ask for an extension to the UK’s EU membership past 31 October in any circumstances, meaning that he will be prepared to defy the UK parliament and drive the UK out of the EU with no deal if an agreement cannot be reached by mid October.

The shadow Brexit secretary Keir Starmer warned the prime minister that if he refused to seek an extension he would be breaking the law and would have to face the consequences. “If the prime minister thinks Parliament is going to stand by while he breaks the law, then he has another think coming,” he said.

Despite the latest defection and a period of extraordinary turbulence at Westminster, Johnson’s determination to defy parliament, and frame the Brexit battle as one between obstructive MPs and the people who voted Leave in the 2016 referendum, seems to be bolstering support for his party.

The latest Opinium poll for the Observer on Sunday shows that the Tories have a 12-point lead over Labour, up two points on 37%. Labour remains on 25%, while the Lib Dems are down one point on 16%. The Brexit party is unchanged on 13%.

Gyimah said that under Johnson the foundations of British democracy were being shaken. “There is a scorched earth approach to delivering Brexit by 31 October at the risk of our institutions, at the risk of our democracy, at the risk of our economy. That should not be allowed to happen.”