Keir Starmer has rubbished Suella Braverman's claim that Brexit had nothing to do with long delays in Dover - and said it was "obvious" there would be problems.
Holiday-makers hoping to get away to Europe at the start of the Easter break have faced misery, with many abandoning their trips after enduring hours of chaos.
Mr Starmer said "of course" Brexit had an impact on the queues - and called on the Government to "get a grip".
It comes after Home Secretary Ms Braverman claimed it was not fair to describe the delays as an adverse effect of Brexit, while No 10 claimed a "combination of factors" was to blame.
The Labour leader told LBC: "Of course Brexit has had an impact.
"There are more checks to be done. That doesn't mean that I am advocating a reversal of Brexit, I am not. I have always said there is no case now for going back in."
But he accused the Government of being ill-prepared, saying: "Once we left, it was obvious that what had to happen at the border would change.
"Whichever way you voted that was obvious. Whichever way you voted you are entitled to have a Government that recognises that and plans ahead.
"Yet again we have got to the first big holiday of the year and we have got queues to the great frustration of many families trying to get out to have a well-earned holiday, and I think my message to the Government, their message, would be get a grip."
Brazen Ms Braverman yesterday told Sophy Ridge On Sunday on Sky News that it would not be fair to view the delays as "an adverse effect of Brexit".
She said: "What I would say is at acute times when there is a lot of pressure crossing the Channel, whether that's on the tunnel or ferries, then I think that there's always going to be a back-up and I just urge everybody to be a bit patient while the ferry companies work their way through the backlog."
She also downplayed any fears that delays at Dover could become a regular occurrence that risks ruining school holiday plans.
Asked whether Brexit was to blame for people having their holidays cancelled, a No10 spokesman said: "I think there are a combination of factors that took place over the weekend.
"It is pleasing that the ports confirmed this morning that all traffic has now been processed and the critical incident has been stood down.
"My understanding is there was a very high volume of coach traffic, there were adverse weather conditions, which resulted in queues..."
Asked again, he said: "I think as you heard this morning... French border authorities are inspecting and stamping every single passport and there were other issues such as the bad weather and higher than expected travel which contributed to these problems.
"Obviously we recognise there are new processes in place. That's why authorities were given a long time to prepare for the new checks, including during the transition period and we are in discussion with our French counterparts over how we can further improve the flow of travel."
Asked a third time whether Brexit is one of the factors that caused disruption, they said: "As I say we know there are new processes in place - we believe we have put in measures to mitigate any of the challenges that may crop up on that.
"That's giving authorities additional time to prepare and working with our French counterparts in increasing our ability to move traffic and passengers through."
This morning former children's minister Tim Loughton told Times Radio that a lack of staff at the French border control is to blame for delays at Dover, not Brexit.
He said: "It's very fashionable to blame everything on Brexit...
"The whole issue about what was going to happen with passports after Brexit is people wanted to make sure we were checking who was going in and out of the country. So the problem isn't with the fact that passports are being more rigorously checked. I think that's a good thing.
"The problem is how many people we've got doing it."
Directing the blame at French officials, he said: "Last time I was going across the channel, we went through the British passport control in a matter of minutes. And then there was a big queue to get through the French passport control because only two booths out of I think 10 or 11 were actually open.
"It's because there were not sufficient border force people there from the French equivalent checking those passports.
"Obviously arrangements have got to be better. There needs to be more people processing those passports at peak time and it needs to flow better but to just blindly say all this is all due to Brexit. It's a bit of a lame excuse."
