(Bloomberg) -- U.K. and European Union officials are discussing a proposal to allow Britain’s post-Brexit transition phase to be extended as a way of breaking the deadlock in talks, according to two diplomats familiar with the situation.

The proposal is to include an extension to the 21-month transition period that’s already due to kick in on Brexit day so that the most controversial part of the divorce deal -- known as the Irish backstop -- is less likely to come into effect. Talks are stuck over the issue of the Irish border and a guarantee, or backstop clause, which is proving almost impossible for Theresa May’s allies at home to accept.

The U.K. government had no immediate comment.

An extended transition would be more palatable to May’s allies in the Northern Irish Democratic Unionist Party as it would apply to the whole U.K. The backstop is controversial because it will treat Northern Ireland differently and erect barriers between the region and mainland Britain.

Both sides in the negotiation say a deal is within reach, but the Irish border -- and how to keep it open after Brexit without erecting barriers between Northern Ireland and Britain -- remains a sticking point. Negotiators are aiming to have an outline deal by next week so that leaders can sign off on a divorce treaty in mid-November.

Under current plans, the transition is to last 21 months after the U.K.’s departure from the bloc in March. It effectively keeps the country as a member of the EU in all but name but strips the U.K. of influence over decision-making. It is designed to give business some predictability while the two sides negotiate a broader agreement on future economic and trading relations.

It is unclear whether negotiators are considering specifying the length of the transition extension in the Brexit deal or whether it would just refer to the possibility of prolongation in future. The diplomats also cautioned that it is only one of several ideas under consideration to unblock talks.

A longer transition would be controversial for May at home. Pro-Brexit lawmakers reluctantly agreed to the grace period after criticizing it as turning the U.K. into a humiliated "vassal" state. They would likely push back against any attempts to lengthen it. However, government officials have privately indicated for months that the transition period probably won’t be long enough.

To contact the reporter on this story: Ian Wishart in Brussels at iwishart@bloomberg.net

To contact the editors responsible for this story: Alan Crawford at acrawford6@bloomberg.net, Emma Ross-Thomas

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