PM warned over price of concessions to Labour

British Prime Minister Theresa May's chief Brexit negotiator is to return to Brussels for talks about changing her draft political declaration on relations between the UK and the European Union after Brexit takes place.

The declaration is a non-binding arrangement which sits alongside the withdrawal terms May agreed with EU leaders last year.

After May tried and failed three times to get those terms approved by Parliament, in desperation she opened cross-party talks with the opposition Labour Party. 

This drew criticism from both Conservative and Labour supporters, and they are thought to have been largely unproductive. Now it is believed that chief negotiator Olly Robbins is going to ask Brussels about the possibility of altering the declaration to include some of Labour's demands, although the government has yet to confirm this.

Labour wants Britain to have a customs union with the EU. This would mean goods transported between Britain and the EU would not be subject to internal taxes, which would help business, but would also prevent the country negotiating its own trade deals with other countries.

Former defense secretary Michael Fallon warned May that any bowing to the demands of Labour leader Jeremy Corbyn are "a blind alley" and told the BBC Radio 4 Today program that no Brexit at all would be preferable to a half-hearted one.

"We can't say we are leaving the EU then half stay in it," he said.

"If they are going to include permanent membership of a customs union then frankly we would be better off staying in the EU then we would have a voice in the trade arrangements that are being negotiated."

A Downing Street source told the BBC that any compromise with Labour over the issue of a customs union would only be a temporary measure.

"We will not sign up to a permanent customs union," said the unnamed source.

"Both sides agree that no Parliament can bind a future government and most EU trade deals have a six to 12-month exit clause."

In addition to Fallon's remarks, May has been sent a letter signed by 13 former Cabinet ministers including Boris Johnson and Dominic Raab, and the chairman of the influential 1922 Committee of Conservative backbench MPs, warning her about the risks of any compromise with Labour.

The Times newspaper reports that the letter says any such move could cost her more Conservative support while failing to gain any from Labour, and cause even greater internal division.

"You would have lost the loyal middle of the Conservative Party, split our party and with likely nothing positive to show for it," said the letter. "No leader can (bind) his or her successor, so the deal would likely be at best temporary, at worst illusory."