The Prime Minister has described concerns about people travelling into Wales for exercise as "hypothetical".
Boris Johnson was asked in the House of Commons by Labour leader Sir Keir Starmer to clarify what would happen if somebody in England travelled towards the Wales border.
After changes were made to lockdown rules in England, which allows people travel anywhere to exercise, he asked: "Can you travel to the border but not beyond the border?
"That makes enforcement extremely difficult."
In response, the Prime Minister said: "There will be myriad hypothetical questions.
"But let's be absolutely clear - I think everybody understands what we are trying to do together.
"And that is working together as a country to obey social distancing rules that everyone understands.
"This is the moment for the whole country to come together, obey those rules and apply their common sense in the application of those rules."
Following his response, Arfon MP Hywel Williams said on Twitter: "Boris Johnson says that people traveling from England to Wales is one of many 'hypothetical' situations.
"It is not hypothetical."
The Welsh Government has made clear that lockdown rules remain in place in Wales, and people are only allowed to exercise locally to their homes.
People who travel into Wales to exercise face fines from police.
Mr Johnson's address to the nation on Sunday evening which outlined changes to measures in England sparked confusion after he failed to say that the changes did not apply to other parts of the UK.
Concerns were raised that people may begin travelling to beauty spots such as Snowdonia as a result of the announcement.
Further guidance released by the UK Government on Monday said: "When travelling to outdoor spaces, it is important that people respect the rules in Scotland, Wales and Northern Ireland and do not travel to different parts of the UK where it would be inconsistent with guidance or regulations issued by the relevant devolved administration."
In a further question, Liz Saville Roberts, Plaid Cymru's Westminster Leader, said: "Can [Mr Johnson] confirm that almost everything he announced today, he is acting as the Prime Minister of England?"
In response, Mr Johnson said he "rejects that completely", before adding that they accepted there could be some differences in rules in different areas.
Responding to a further question, he added that people should not take the changes to lockdown measures in England as meaning people can go to holiday homes or second homes elsewhere in the country.
