Mr Opperman campaigned for the UK to remain in the EU and gave David Cameron his full support, but says following the result, it is important that the PM's successor is someone who can get the best deal for Britain in the coming negotiations.
Mr Opperman said: "The key priority for my constituents now is ensuring economic stability, and getting the best deal for Britain. Theresa stands head and shoulders above the others in being able to achieve that. Having worked first hand alongside Theresa I think she is the natural successor to lead our country through these difficult and uncertain times.
"I worked for Theresa in the Home Office for nearly 3 years and I saw for myself, day in and day out, how hard she worked to keep our country safe and secure. No one gets up earlier. No one is a better master of her brief. Quite frankly, no one works harder. Everyday she has been working hard to keep our country safe.
"Just look at her national security credentials - she has spent six years in charge at the Home Office running our police and security services and combating the threat from terror groups. Theresa clearly has the serious experience of high office to deliver the best deal in the EU negotiations. That kind of experience can not be underestimated.
"Theresa also understands the challenges we face in the North East, having cut her political teeth fighting a Durham seat in the 1990's. She talked in her launch speech of understanding the concerns facing an ordinary, working-class family, setting out how life for many is much harder than many people in Westminster realise.
"She talked about job security, the worry of rising mortgage rates, the cost of living and the quality of local schools. It was a speech about real issues, facing real people. Fundamentally she understands the issues that matter to the people of the North East.
"Theresa understands that politics isn’t a game, it is serious, and has real consequences for people’s lives.
"In truth, I thought long and hard about all of the candidates in the race, some of who are friends of mine, and are people I greatly admire. However I wanted to make a decision, not just on who I thought would be the best leader for the Conservative Party, but for the North East as a whole.
"That is why I am supporting Theresa to become our next Prime Minister.
"I am backing Theresa because of her commitment to put the Government she will lead at the service of ordinary, working people and make Britain a country that works for everyone – regardless of who they are and regardless of where they’re from.
"Theresa would make an excellent Prime Minister. She is the serious politician we need for these serious times."
Voting on the future Prime Minister will take place on Tuesday of this week.
MPs will whittle down the list of candidates to two by the middle of next month. A ballot of the party’s 150,000 members will then follow, with a new Tory leader named by September 2.