Toyota announced in March that it would install its new car platform at its plant in Burnaston, Derbyshire.
The Department for Business, Energy and Industrial Strategy (DBEIS) has confirmed to the Reuters news agency that the letter exists, but has refused to release it and comment further.
A source told Reuters the letter was similar to one sent to fellow Japanese firm Nissan last year, when it decided to build two new models at its Sunderland plant.
The letter provoked concerns about the carmaker being offered a “sweetheart deal” by the Government.
Business Secretary Greg Clark, who has refused to release the Nissan letter, said last year that the assurances offered to Nissan were available to other companies.
On the subject of the Toyota letter, the Reuters’ source said it contained a number of assurances, but did not say when it was sent.
“They received a similar set of warm words as Nissan on electric vehicles, commitment to further training and to ensure the competitiveness of the UK automotive industry,” they said.
A spokesman for Toyota declined to say whether the firm had received such a letter.
He referred to the company’s statement in March, which said the Government was providing the company more than £21m for training and research and development.
Toyota also said at the time that “continued tariff-and-barrier free market access … will be vital for future success”.
Reuters submitted Freedom of Information requests to DBEIS asking for documents relating to the Toyota decision to be released.
In its response, the department confirmed there was a letter, but refused to release it along with a company briefing note.
It said the information was “both highly commercially sensitive” and “would be likely to cause harm to the company’s commercial interests if disclosed”.
The department did however reveal that Mr Clark had meetings with Toyota officials in September, November and March.
The Brexit vote has left a number of the world’s biggest car firms worried about the long-term viability of their British operations.
Many are pushing the Government for promises to maintain free trade after Brexit takes effect in March 2019.
Nissan, Toyota and another Japanese company, Honda, account for around half of British car production.
Japanese Prime Minister Shinzo Abe met Prime Minister Theresa May in April and called for a smooth Brexit in order for Japanese firms to be able to continue to operate.