On Friday, Reuters cited two sources saying that Britain had helped to secure an investment in the firm's Burnaston car plant with a letter reassuring the Japanese automaker over future trading arrangements.
The business ministry has confirmed the existence of a letter but has refused to release it.
One of the sources said the letter to Toyota was similar to one sent to Japanese carmaker Nissan last year when it decided to build two new models at its northern English plant.
The previous chairman of parliament's Treasury Select Committee raised questions about the Nissan deal last year. Reuters contacted the new committee chairwoman on Monday for comment on the reported reassurances to Toyota.
"To provide clarity to the public, as the assurances may cost the taxpayer money, and to other businesses, who are craving certainty to plan for Brexit, the letters should be published immediately," Nicky Morgan said in an emailed statement.
"It remains to be seen what these assurances could have been that were robust enough for them to invest in the UK, but avoided any obligation to report to parliament," she said.
A Toyota spokesman referred to the company's statement from the day of the March 16 investment announcement which said the British government was providing up to 21.3 million pounds in funding for training and research and development.
The chairwoman of parliament's Business Select Committee, opposition Labour lawmaker Rachel Reeves, said it was unacceptable that the government had refused to reveal its private reassurances to Toyota.
"It is vital that the government is not seen to be cutting sweetheart deals or granting special favors that could undermine our negotiating position," she said in a statement.