A spokesperson for the Electoral Commission said it was seeking to establish whether any sums relating to the refurbishment should have been declared under the law on party political donations.
In response, the Conservative Party said that all reportable donations were correctly declared in compliance with the law.
It follows reports by the Daily Mail that £60,000 of party funds were used to help cover the reported £200,000 cost of refurbishing the official flat over 11 Downing Street where Mr Johnson lives with his fiancee, Carrie Symonds.
In a statement, the commission said: “We are in contact with the party to establish whether any sums relating to the renovation works fall within the regime regulated by the commission.
