THERESA May has arrived in Paris where she will urge the French president Emmanuel Macron to back her case for a delay to Brexit at a crunch EU summit on tomorrow.

The Prime Minister travelled from Berlin where she met German chancellor Angela Merkel for a "working lunch" over which they agreed on the importance of an "orderly withdrawal" from the EU, Downing Street said.

The visit got off to an embarrassing start for the Prime Minister as no-one was there to greet her when she arrived for the negotiations.

WATCH: Theresa May snubbed in Berlin as she arrives for Brexit talks

In recent days Macron has warned that an extension to the Article 50 withdrawal process is not guaranteed.

The quick-stop visits come as the EU's chief Brexit negotiator, Michel Barnier, said Brussels could amend the Political Declaration on future relations with the UK "within a few hours or days" to incorporate the customs union arrangement being discussed in cross-party talks between the Government and Labour.

READ MORE: Brexit crisis as talks between May and Corbyn flop

But there were signs of resistance in May's Cabinet to compromise with Labour, with International Trade Secretary Liam Fox warning that a customs union would leave the UK "stuck in the worst of both worlds".

May has asked for the date of Brexit to be delayed until June 30 at tomorrow's summit, with the possibility of an earlier departure if the UK's withdrawal deal is ratified.

But European Council president Donald Tusk is recommending a year-long delay, while French Europe minister Amelie de Montchalin suggested Paris may insist on conditions limiting British influence on EU decision-making during the extension period.

The unanimous agreement of all 27 remaining EU states is needed to avoid a no-deal Brexit on the scheduled date of April 12.

De Montchalin warned that "the position of France has not changed" and that the EU27's approval of May's request is "neither settled nor automatic".

"We want to understand what the UK needs this extension for," she said. "Then comes the question of the conditions – what role the UK wants to play during this extension, on what type of decisions it wants to play a role."

In a clear sign of opposition to concessions to Labour's demands, Dr Fox sent a letter to the chairman of the Tory backbench 1922 Committee setting out in detail his opposition to a post-Brexit customs union.

The letter, obtained by the Daily Telegraph, warns that such an arrangement would leave the UK unable to set its own trade policy but forced to open its markets to any country with which the EU struck a free trade agreement.

"In such a scenario the UK would have a new role in the global trading system – we ourselves would be traded," he told Graham Brady.

"As the famous saying in Brussels goes, if you are not at the table, you are on the menu."

Leader of the Commons Andrea Leadsom urged May to ask Merkel to re-open the Withdrawal Agreement forged with the EU last November – something the EU has repeatedly said it will not do.

Leadsom said: "The Prime Minister is off to see Angela Merkel today and it would be fantastic if Angela Merkel will try to support a proper UK Brexit by agreeing to reopen the Withdrawal Agreement.

"There have been rumours over the weekend some senior members of the German government would be willing to do that in order to get Theresa May's deal over the line.

"As the person with the responsibility to get the legislation through, if we get the Prime Minister's deal over the line because the EU has decided to support measures on the backstop, that would be the best possible outcome."

But May's official spokesman dismissed the idea, telling reporters: "Any plan going forward would be based on the current Withdrawal Agreement."