The Prime Minister’s comments come as Downing Street revealed on Tuesday that sending migrants to Rwanda may not take place for months.
Number 10 was unable to say when the first flights sending migrants on a one-way trip to the East African nation would take off, in the face of criticism and legal action against the plans.
Officials insisted they were not waiting for court challenges to be resolved before putting the policy into practice.
Speaking to reporters at Southampton Airport on Wednesday, Mr Johnson claimed “we always knew” that “liberal lawyers” would try to make the plan difficult.
However, he stressed the partnership between Rwanda and the UK is a “very, very sensible thing”.
The Prime Minister said: “It’s a great deal between two countries, each helping the other.
“Of course, there are going to be legal eagles, liberal lawyers, who will try to make this difficult to settle. We always knew this was going to happen, but it is a very, very sensible thing.
“If people are coming across the Channel illegally, and if they are, their lives are being put at risk by ruthless and unscrupulous gangsters, which is what is happening at the moment.”
He added: “You need a solution. And you need something that is going to say to those people, to those gangsters, ‘I’m sorry, but you can’t tell your customers, you can’t tell these poor people, that they’re just going to come to the UK, and they’re going to be lost in the system, because we’re going to find a way of making sure that they are going immediately to Rwanda’.
“I think that’s a humane, compassionate and sensible thing to do. I’m not going to pretend to you that is going to be without legal challenges. I think I said that when I announced it, but we will get it done.”
Earlier this month, Home Secretary Priti Patel signed what she described as a “world-first” agreement with Rwanda, which will see the East African nation receive asylum seekers deemed by the UK to have arrived “illegally” and therefore inadmissible under new immigration rules.
The deal is already subject to legal challenges.
The Prime Minister’s official spokesman said the flights would take place at the “earliest opportunity” and the plan was a “fully legally secure approach that has been tested and thought through”, adding: “We have received pre-action correspondence from a number of legal firms, I can’t get into that more… but we still maintain our hope to have the first flights take place in a matter of months.”
Plans for an immigration processing centre in North Yorkshire, which are said to be pivotal to “delivering” the Government’s Rwanda deal, have also been criticised by Labour.
Asked by reporters about plans for the processing centre at a former RAF base near Linton-on-Ouse, party leader Sir Keir Starmer said: “The Government is in complete chaos on this.
“To call it a plan is too grand: they don’t know what they are doing, they haven’t thought it through and they haven’t even had the decency to consult local people about it, which tells you just about everything you need to know about the state of chaos they are in.”
I. Stephanie Boyce, president of the Law Society of England and Wales, said: “The PM appears to have little confidence the plan to send asylum seekers to Rwanda is legal if he is even at this early stage preparing for it to be challenged in the courts.
“It is not clear that the Nationality and Borders Act gives the Government the powers to do this.
“There are questions as to whether this scheme would comply with the Refugee Convention, whether refugees threatened with being sent to Rwanda would be able to access justice if they are mistreated.”