The government will publish its economic analysis on the long-term effects of Brexit in the UK.
Scenarios will be set by the Ministry of Finance – with the Daily Telegraph saying it will predict £ 150bn in output lost for 15 years under no agreement, with Theresa May's plan worth £ 40bn.
Meanwhile, the PM will visit Scotland and believes he is "strong" in defending fishing in Britain during his Brexit talks.
The SNP claims that the industry will be "sold out".
The party said access to British waters for EU ships would be used as a "bargaining chip" to secure a good post-Brexit trade deal.
However, Mrs May will continue to maintain the Brexit withdrawal agreement which she agreed with the European Union to secure "full state control of our waters" in Britain and is the best way to protect the economy.
According to the Daily Telegraph, Treasury analysis will show that under Ny May's agreement, UK GDP will be between 1% and 2% lower for 15 years than if it remained in the European Union, compared with 7.5% lower under no there is a situation agreement.
The European Union Exit Department said it did not comment on leaks.
The Telegraph also quoted former Brexit Secretary Dominic Raab as saying the Ministry of Finance analysis "looks like a repeat of the Fear Project".
In other developments, the Commons Public Accounts Committee issued a report warning of the "real prospects" of "major disruptions" in British ports in the Brexit no-deal case.
Lawmakers will vote on Mrs May's Brexit agreement, which she insisted was the only option, on December 11.
Against this background, the prime minister visited various parts of Britain to be directly involved with the public and business in an effort to build support for the agreement.
He insisted it was protecting "vital interests" from across Britain when visiting Wales and Northern Ireland on Tuesday, while rejecting claims from DUP leader Arlene Foster that he had "surrendered" to negotiations before agreeing to the Brexit agreement.
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During a visit near Glasgow, Mrs May will talk to factory workers about how she believes that her agreement provides "unprecedented economic relations that other large economies do not have" and certainty for their employers and staff.
"We will be free to conduct our own trade transactions throughout the world, providing greater opportunities for Scottish exporters," he said.
He will also say that moving away from the EU's general fisheries policy (which has tragically disappointed Scottish coastal communities "will leave Britain" free to decide for ourselves who we allow to fish in our waters ").
Britain sells nearly £ 1 billion of fish products to the European Union every year, and a number of EU countries insist that such free trade rates can continue only if EU fishing vessels continue to gain access to British waters.
Scottish Conservative Ross Thomson, has voiced concern the draft agreement could lead to "continuation of some forms of CFP".
And Scottish First Minister Nicola Sturgeon has revealed an analysis of SNP claims that Scotland will be poorer with the agreement.
His party, which has 35 members of parliament, along with the leadership of the Labor Party, Lib Dems and Unionist Democrats, all said they would reject the agreement of Mrs. May.
Many Tories also said openly that they were opposed to Mrs. May's agreement.
Legal advice line
Meanwhile, a dispute is occurring after Labor demanded the government publish its full legal advice on the Brexit agreement this week.
The government only said it would publish a "reasoned position statement" which laid its political and legal position on the withdrawal agreement.
BBC Newsnight political editor Nick Watt said a source said the complete advice provided a "very strong warning" that it would not be possible for Britain, by itself, to be able to get out of what is called the Northern Ireland backstop.
The backstop – a plan to create a single temporary customs area to prevent the return of customs posts on the Irish border if there is no EU-UK trade agreement in effect – is still controversial because Brexiteers fears that this will make Britain bound to unlimited EU rules.