Boris Johnson pledges to strengthen Union as he prepares for clash with Ruth Davidson

It comes as Ruth Davidson prepares for a Brexit showdown with Mr Johnson after she insisted she will not support leaving the European Union without a deal.

The Scottish Tory leader stressed she is free to act “independently of government”, and does not have to sign up to Downing Street policy.

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Mr Johnson is due to hold talks with Ms Davidson in Edinburgh, but her latest comments further highlight the gulf between them.

They also came as Michael Gove, the minister responsible for no-deal preparations, said the UK Government is now operating on the assumption that Britain will leave without an agreement.

Writing in the Mail on Sunday, Ms Davidson said she had supported Theresa May’s efforts to get her Brexit deal through Parliament.

She added: “There are huge challenges in changing that deal but I hope beyond measure the new Prime Minister is successful in getting an agreement with the EU so he can go back to the House of Commons and get the majority backing he needs. He has my full support in those efforts.

“Where I differ with the UK Government is on the question of a no-deal Brexit.

“When I was debating against the pro-Brexit side in 2016, I don't remember anybody saying we should crash out of the EU with no arrangements in place to help maintain the vital trade that flows uninterrupted between Britain and the European Union.

“I don't think the Government should pursue a no-deal Brexit and, if it comes to it, I won't support it.”

She added: “As leader of the party in Scotland, my position exists independently of government. I don't have to sign a no-deal pledge to continue to serve.”

Ms Davidson said she would “not be backward in challenging Mr Johnson's Government where I think they are getting it wrong – and Boris is more than welcome to do the same to me when he thinks I'm wrong too”.

However, she rejected any suggestion the Scottish Tories might split from the UK party – an idea previously mooted eight years ago.

Writing in The Sunday Times, Mr Gove said the Government is now "operating on the assumption" that Britain will leave the EU without a deal.

Camley's Cartoon

He said: "With a new prime minister, a new government, and a new clarity of mission, we will exit the EU on October 31. No ifs. No buts. No more delay. Brexit is happening.”

Meanwhile, Chancellor Sajid Javid said there will be "significant extra funding" this week to get Britain "fully ready to leave" the EU on October 31, with or without a deal.

The additional spending will include financing one of the country's "biggest ever public information campaigns".

Mr Javid also plans to fund 500 new Border Force officers, as well as looking at new infrastructure around the country's ports to minimise congestion and ensure goods can flow.

Labour's shadow Brexit secretary Sir Keir Starmer said he was working on a cross-party alliance to prevent no-deal.

Ms Davidson has long been seen as one of Mr Johnson’s key critics within the Tory Party and backed his rivals in the leadership race.

Their relationship was dealt a further blow last week when the new Prime Minister sacked her close ally David Mundell as Scottish Secretary, replacing him with Alister Jack.

Scottish Tory insiders have previously raised fears Mr Johnson’s premiership could boost support for independence.

SNP MP Stephen Gethins insisted Ms Davidson must now set out exactly how she will meaningfully oppose Mr Johnson's no-deal Brexit plans – including by giving a cast-iron commitment that all Scottish Tory MPs will vote against it.

He said: “It’s time for Ruth Davidson to find a backbone and join the SNP in meaningfully opposing Boris Johnson’s disastrous Brexit plans – instead of always rolling-over. She says she will support Mr Johnson but not a no-deal Brexit – but the fact is you can’t do both. Her position is untenable and weak.”

Mr Johnson will make his first official visit to Scotland today, where he will announce the expansion of Growth Deals across Scotland, Wales and Northern Ireland, backed by £300 million of new funding.

In Scotland, this will see extra investment pumped into Falkirk, the islands and Argyll and Bute. Similar deals – which the UK Government has committed £2.78bn of funding towards – have kickstarted a number of projects in Scotland’s cities, including a new 1000-seat concert hall in Edinburgh.

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Mr Johnson said he will work closely with devolved governments to put in place such schemes in every region of Scotland, Wales and Northern Ireland.

He said: “Our Union is the most successful political and economic union in history. We are a global brand and together we are safer, stronger and more prosperous.

“So as we prepare for our bright future after Brexit, it’s vital we renew the ties that bind our United Kingdom.

“I’m proud to be in Scotland today to make clear that I am a passionate believer in our great Union, and I look forward to visiting Wales and Northern Ireland to ensure that every decision I make as Prime Minister promotes and strengthens our Union.”

He added: “As I said when I stood on the steps of Downing Street last week, it is time that we unleashed the productive power not just of London and the South East but of every corner of England, Scotland, Wales and Northern Ireland.

“Important projects like government’s Growth Deals – today backed with £300m of new funding – will open up opportunities across our Union so people in every corner of the United Kingdom can realise their potential.”

A spokesman for SNP Finance Secretary Derek Mackay said the £300m fell "far short of what is required".

He said the UK Government is lagging behind the Scottish Government's funding of Growth Deals, and accused Mr Johnson of "fiddling while Rome burns".

He added: “If Mr Johnson thinks he can win over Scotland with recycled and underfunded spending plans he has a lot to learn in his new role.”