The Prime Minister said there will be "some opening up" on May 17, but that things must be done in a way "to make sure that we don't see the virus coming back in" to the UK.
Mr Johnson's cautious tone came as some MPs called for restrictions on foreign holidays to be maintained to protect the country from Covid-19 variants, and Labour leader Sir Keir Starmer urged a "careful" approach.
Mr Johnson told reporters during a campaign visit to Hartlepool: "We do want to do some opening up on May 17, but I don't think that the people of this country want to see an influx of disease from anywhere else.
"I certainly don't and we have got to be very, very tough, and we have got to be as cautious as we can, whilst we continue to open up."
Asked if people should be planning foreign holidays, he said: "We will be saying more as soon as we can.
"I think that there will be some openings up on the 17th, but we have got to be cautious and we have got to be sensible and we have got to make sure that we don't see the virus coming back in."
Sir Keir criticised the "chopping and changing" of the travel corridors list introduced last year and said such a situation should be avoided this holiday season.
Speaking during a campaign visit to Lewisham on Monday, the opposition leader said he will "wait and see" what the UK Government announces.
He said: "It's clear that the virus is increasing in some countries around the world, so we have to be very, very careful.
"What we can't have is a repeat of last summer, where the lists were chopping and changing on a daily or even weekly basis."
May 17 is the earliest date international travel could be permitted under the UK Government’s road map for the easing of lockdown.
First Minister Nicola Sturgeon has previously said non-essential international travel from Scotland was unlikely to resume from May 17.
It comes as the European Commission proposed to ease restrictions on travel to countries in the bloc amid progressing vaccination campaigns and lower infection rates.
The commission said it was proposing "to allow entry to the EU for non-essential reasons not only for all persons coming from countries with a good epidemiological situation, but also all people who have received the last recommended dose of an EU-authorised vaccine".
It was not said which countries would be on its list, but one unnamed EU official said the UK remains a "question mark".
One travel firm boss said there had been "great progress" in countries such as Portugal and Spain in preparing for the return of holidaymakers.
Thomas Cook chief executive Alan French told BBC Radio 4's Today programme: "When the holidays proper start at the end of June, we are expecting most of the countries that the UK goes on holiday to – Europe particularly – to be open.
"We are expecting Portugal, Spain, Greece, Croatia and so forth to be open, it would be nice if Turkey was open."