Labour leader Sir Keir Starmer has called for Boris Johnson and Wales' first minister to reach agreement on travel restrictions.
He said Mark Drakeford, who plans a ban on travel from Covid hotspots to Wales, was "trying to keep Wales safe".
Sir Keir initially said he hoped "it doesn't come" to restrictions, but after he was asked several times for his position said he would support them if it controlled coronavirus.
The ban is set for Friday at 18:00 BST.
Mr Drakeford, the Welsh Labour leader, says he would bring it into force if the prime minister does not introduce his own restrictions.
The Welsh Labour leader has written twice in recent weeks to Mr Johnson asking for travel to be restricted in and out of areas with high levels of transmission in England.
"What Mark Drakeford is trying to do is to keep people in Wales safe," Sir Keir told BBC Wales.
"He is frustrated. He's been asking the prime minister to work with him on this.
"Mark's objective is to keep people safe and he's absolutely right about that.
"I think the ball is really in the prime minister's court to do something about it.
"Because I hope it doesn't come to this... travel restrictions. I don't think it needs to if the prime minister enters discussions with Mark in the right spirits."
Sir Keir added: "The prime minister just needs to be clear that people shouldn't travel from high infection rate areas into areas in Wales where there aren't those high infection rates."
After he was asked several times for his position on what should be agreed between the two governments, Sir Keir said: "There has to be a way of controlling the virus.
"And if that unfortunately means people can't travel in the way they did before then that's got to be put in place."
'Stirring division and confusion'
About 2.3 million people in 17 areas of Wales are already subject to a bar on travel, with exceptions for school, work and other limited reasons.
It is expected the ban on people from Covid hotspots elsewhere in the UK will have similar exceptions.
The rules will be imposed on people from Tier 2 and 3 regions in England, the whole of Northern Ireland and the Scottish central belt.
The UK government's Welsh secretary has warned the first minister's travel restrictions plan risks "stirring division and confusion".
Simon Hart questioned the enforceability of the measures and asked for further detail on how the restrictions will work, and what exemptions there might be.