Steel magnate Mittal -- who has donated more than four million pounds to Labour -- sat next to Blair at a meeting of "powerful" international businessmen in Kazakh capital Astana in May, the Daily Mail said.
Mittal is Britain's richest man with a fortune of more than 20 billion pounds (USD 32 billion) and is the biggest employer in Kazakhstan. He employs 50,000 people there and is reportedly close to Kazakh President Nursultan Nazarbayev.
Mittal's LNM steel company is, however, not British. It was registered in the Dutch Antilles and has less than one percent of its 100,000-plus workforce in Britain.
An adviser to the Kazakh president has said Blair opened an office in the capital.
"A large working group is here and, to my knowledge, it has already opened Tony Blair's permanent office in Astana," said presidential adviser Yermukhamet Yertysbayev.
Kazakhstan is desperate to improve its image, which has been mocked as underdeveloped.
In leaked US diplomatic cables, Kazakhstan has been accused of being undemocratic, using torture and other abuse, arbitrary arrest and detention, restriction on free speech, corruption in law enforcement and the judicial system, discrimination and violence against women and people trafficking.
Despite this, Blair and close colleagues have made a number of visits there, said the Daily Mail.
Blair met Nazarbayev Jan 31, and reportedly discussed the establishment of new financial institutions.
"Tony Blair has helped put together a team of international advisers and consultants to set up an advisory group for the Kazakhs, with a team of people working on the ground. The work they are doing is excellent, sensible and supports the reforms they are making," said a spokesman for Blair.
"The Kazakhs also engage with a number of other former European leaders. Blair is not personally making a profit directly or indirectly through Tony Blair Associates or any company on this. He is not doing business in Kazakhstan," he said.
A spokesman for Mittal has declined to comment.
Mittal's employees in Kazakhstan have accused him of "slave labour" conditions after a series of coal mining accidents between 2004 and 2007 which led to 91 deaths.