May flew to Brussels, Belfast and Dublin last week for Brexit talks.
European Union leaders stuck to their guns on Wednesday (30 January), insisting that the Brexit Withdrawal agreement rejected by UK MPs would not be renegotiated.
The Leave campaign figurehead was speaking as Brexit Secretary Stephen Barclay prepared for talks with European Union chief negotiator Michel Barnier as the Government stepped up efforts to secure changes to the backstop, which is created to keep the Irish border open after the UK's withdrawal.
Their teams would continue to work in the meantime "on finding a way forward", it added.
"I found Corbyn's letter interesting in tone and in content", Barnier said earlier in the day in Luxembourg of the Labour leader's Brexit proposals, sent to May.
This week's Brexit votes may not come to much.
British lawmakers overwhelmingly rejected the deal, agreed in November, in a vote last month, leaving Britain on course for an abrupt Brexit with no transition period to minimise expected economic disruption.
I know that sounds nebulous, a word you might choose to use.
British Prime Minister Theresa May has rejected the idea of targeting a customs union with the European Union, pouring cold water on hopes from some that she could shift her Brexit policy to win over the opposition Labour Party.
The existing Political Declaration - the part of the Brexit deal setting out the goals for the future UK-EU relationship - "explicitly provides for the benefits of a customs union - no tariffs, fees, charges or quantitative restrictions across all sectors and no checks on rules of origin", said the PM.
And she questioned whether Mr Corbyn's call for "frictionless" trade would mean remaining in the single market and reneging on Labour's commitment to end free movement.
Mr Corbyn has repeatedly said there should be an election if Mrs May can not get a deal through Parliament and he has faced concerted pressure from some in his party to push for a second public vote. He has also faced pressure from some of his MPs to push for another public vote on Brexit.
Keir Starmer, who is Labor's shadow Brexit secretary, says he believes the prime minister is "pretending to make progress" on the Irish backstop issue.
"I don't see that in the letter", he said.
"On your point about the deadline and could we have a time limit, I think it must be pretty obvious that if you are going to have a time limit to the backstop and I think that would be a very good thing".
Laughing as he spoke, he said: "It is, in fact, the same protagonists for Brexit and a no-deal who do not have the courage either to put the issue to a peoples' vote". And how to make sure that the United Kingdom isn't locked in that prison of the customs union.
Speaking to BBC radio Monday morning, Johnson said: "The argument is now about how to get out of the backstop".
Members of the Alternative Arrangements Working Group - including Conservative MPs Steve Baker, Marcus Fysh, Owen Paterson, Damian Green and Nicky Morgan - attended the meeting.
UK To Send F-35s, Aircraft Carrier to South China Sea
Last year, China added surface-to-air missiles to the contested island, one of three that the Chinese military fortified in 2018. Aerial view on the The Spratly Islands, one of the major archipelagos in the South China Sea .