LONDON (Reuters) - The British government must show lawmakers the full legal advice it has received on the so-called Irish backstop before they are asked to vote on any Brexit agreement, the opposition Labour Party said on Wednesday.

Disagreements over the backstop, an insurance policy to avoid a hard border between British-ruled Northern Ireland and EU member state Ireland, remain the key stumbling block to reaching an exit deal with the European Union.

London wants the backstop to have a clear end date or exit mechanism, to ensure it is not tied to the EU’s customs union indefinitely, but Ireland has said Britain cannot be allowed to withdraw from any agreement unilaterally.

Local media have reported Attorney General Geoffrey Cox has drawn up advice, at the request of Prime Minister Theresa May, on the various options available for a potential backstop.

Labour’s Brexit spokesman Keir Starmer, who is in Brussels to meet representatives from EU institutions, said any agreement on the backstop must be “robust and credible”.

“It’s essential MPs (members of parliament) are given the opportunity to scrutinize the Attorney General’s legal advice before voting on the final deal,” he said in a statement.

“At this crucial stage, parliament should not be kept in the dark nor should the government try to bounce MPs into an agreement without all the facts.”

Prominent Brexiteer and environment minister Michael Gove asked May at a meeting of her top team on ministers on Tuesday whether they could be shown the full legal advice, rather than just being shown a summary, according to media reports.

May’s spokesman said the government does not comment on legal advice.

Earlier on Wednesday, a senior lawmaker in the Northern Irish party that props up May’s Conservatives in parliament said it would be in the public interest for the government to publish its legal advice on the backstop.

Former Brexit minister David Davis, who resigned in July over the government’s Brexit plans, has also called on the government to make the advice public.

“We need the cards laid on the table so we can form a judgment. Is the future of the union at stake?,” he wrote in the Sunday Times. “Does ‘temporary’ actually mean ‘eternal’ when it comes to the backstop?”