During Wednesday's Prime Minister's Questions, he repeated his previous suggestion that the UK government would "unblock the Brynglas tunnels" with a "proper M4 bypass".
But with road building in Plaid Cymru's Liz Saville Roberts said Mr Johnson had shown "complete ignorance of how devolution works and for the environment too for that matter".
When asked about the prime minister's comments, Mr Hart acknowledged "it is a devolved matter, it is a matter for Welsh Government to do that".
"UK government has said it would provide the borrowing facilities, the ability to do that. But consistently Welsh Government have declined that offer and so we are at that rather stalemate period."Wales the responsibility of the Welsh Government in Cardiff, a spokesperson said Mr Johnson could not have a say on it.