The lockdown will begin at 6pm on October 23 and last until November 9, and will be similar to the measures placed on the UK at the start of the pandemic.
For the two weeks the measures are in place, pubs, restaurants, hairdressers and gyms will all close, with only essential shops - for example pharmacies and supermarkets - allowed to remain open, the letter suggests.
A final decision on whether to implement the lockdown or not is expected to be made by the Welsh government on Monday.
Health is a devolved issue, so any measures brought in in Wales would not apply to the rest of the UK.
The details of the plan were contained in a letter from a regional director of the Confederation of Passenger Transport to its members in Wales, which was posted on Twitter.
In the letter, dated October 16, John Pockett writes: “The Welsh Government will announce on Monday a ‘circuit break’ lockdown to begin at 1800 on Friday 23 October and continue in force until 0001 on Monday 9 November.
“We have met with officials this morning, but as this is a very fast moving situation with decisions still to be made by ministers, much of the detail has not yet been agreed by the Government.
“Nevertheless I wanted to let you know what we know so far.”
Mr Pockett said the lockdown would “take us back to the situation in March” when pubs, cafes, restaurants and hairdressers were closed.
“It covers the half-term break (Friday 23 October – Monday 2 November) but some schools will reopen on 2 November,” he wrote.
“Ministers have not yet determined the details on this; it seems that primary schools will reopen, but a decision on secondary schools (or at least some or part of individual schools) will be made over the weekend.”
He said public transport would be for “essential journeys only” and the Welsh government was yet to decide what level of services would run during the lockdown.
The letter added: “I appreciate that this information is very limited and I am sure that you will have questions, but that, at the moment, this is very much as we know.
“I assure you that we will keep members fully informed as we receive more details from the government.”
ITV News Wales Correspondent Rupert Evelyn said the letter had been confirmed as true to him.
Mr Pockett told the Press Association he was "surmising" what could happen after he attended a meeting with officials.
“The letter is genuine and it contains what I assume or surmised would be the position.
"It was me advising my bus operator members to be prepared for something and this is what it may well be,” he said.
“It could be more; it could be anything. I think other associations have communicated with their members in the same way.”
Asked about the specific details of the lockdown, Mr Pockett said: “I’ve not seen the detail either – that’s me surmising – but that’s what my common sense tells me.
“I understand what the government wants to do is cause as little disruption to schools as possible, so you start it as soon as the schools finish for half-term.”
On Friday, First Minister Mark Drakeford said the Welsh Government was looking “very carefully” at introducing a circuit-breaker lockdown with a decision due to be announced on Monday.
Tougher coronavirus measures are in force in 17 areas of Wales affecting more than 2.3 million people but had not slowed the spread of the virus enough, he said.
A travel ban preventing people from areas of the UK with high levels of coronavirus from entering Wales came into force on Friday.
“The measures we have put in place at both a local and a national level, with help from the public, have kept the spread of the virus under check,” a Welsh Government spokesperson said.
“However, there is a growing consensus that we now need to introduce a different set of measures and actions to respond to the virus as it is spreading across Wales more quickly through the autumn and winter.
“We are actively considering advice from Sage [the government's Scientific Advisory Group for Emergencies] and our TAC Group.
"A ‘fire break’ set of measures to control Covid-19, similar to that described in the Sage papers, is under consideration in Wales.
“As the First Minister set out in his press conference on Friday, we have discussed this advice with stakeholders and partners. But no decisions have been made.”
Conservative shadow health minister Andrew RT Davies said: “The First Minister knew about this yesterday, and while we all suspected this was going to happen, there was no point in delaying it being announced.
“Instead, he chose to bypass the Welsh Parliament and members not of his party, and first brief outside organisations about it.
“It shows incredible disrespect to the institution of the Welsh Parliament, the voters, and it demonstrates the downright and breath-taking arrogance we have come to expect from the First Minister and the Labour Party.
“Worse, they have actually misled the public and Members of the Welsh Parliament by saying yesterday that ‘no decisions had yet been taken’, when clearly they had.
“Labour’s contempt for democracy and the Welsh electorate is staggering, and I have written to the Llywydd to express or grave concerns over the First Minister’s actions.”