BRUSSELS — British Prime Minister Theresa May came under attack from across Britain's political spectrum Thursday after saying she's considering a European Union proposal that would keep the UK bound to the bloc's rules for more than two years after it leaves in March.

Seeking to unblock Britain's stalled divorce talks with the EU, May said a proposed 21-month transition period for the UK after Brexit could be extended by "a matter of months." At present, the two sides say Britain will remain inside the EU single market, and subject to the bloc's regulations, from the day it leaves on March 29 until December 2020, to give time for new trade relations to be set up.

But with Brexit talks at an impasse, the bloc has suggested extending that period, to give more time to strike a trade deal that ensures the border between Northern Ireland and the Republic of Ireland remains friction-free — the main sticking point to a Brexit deal.

May said the UK was considering an extension of several months. But she said the extra time was merely an insurance policy and was unlikely to be needed.

"We are working to ensure that we have that future relationship in place by the end of December 2020," May said as she arrived Thursday at EU headquarters in Brussels for meetings on migration, security and other issues.

The extension idea angered pro-Brexit UK politicians, who saw it as an attempt to bind Britain to the bloc indefinitely.

In an open letter Thursday to May, leading Brexiteers accused the EU of "bullying" and said the border issue was being used as "a trap" by the bloc. The letter signed by former British Foreign Secretary Boris Johnson, ex-Brexit Secretary David Davis and other pro-Brexit Conservatives warned May not to "engage in a show of resistance and a choreographed argument followed by surrender" to the EU.

Pro-EU politicians, meanwhile, said the proposal was another sign of May's weak bargaining hand and an attempt to stall for time. Liberal Democrat lawmaker Tom Brake said May was merely "kicking the can further down the road." Divorce talks between Britain and the bloc have stalled on the issue of the Irish border, which will be the UK's only land frontier with the EU after Brexit. Both sides agree there must be no hard border that could disrupt businesses and residents on both sides and undermine Northern Ireland's hard-won peace process. But each has rejected the other side's solution.

The EU says the solution is to keep Northern Ireland inside a customs union with the bloc, but Britain rejects that because it would mean customs checks between Northern Ireland and the rest of the UK.

Britain has proposed instead that all of the UK could stay in a customs union — but only temporarily. The EU insists there can be no time limit.

The two sides remain deadlocked, and this week's summit, which had been billed as a make-or-break moment, turned simply into a chance for Britain and the EU to give themselves more time — perhaps until the end of the year — to break the logjam.

May urged both parties to show "courage, trust and leadership," but came to Brussels without the concrete new proposals the EU has asked for. Chief EU negotiator Michel Barnier said "we need much time, much more time, and we continue to work in the next weeks." The lack of progress means a special EU summit on Brexit that had been penciled in for next month has been scrapped, though EU leaders said they would assess the situation later.

The next official EU summit is scheduled for December, just over 3 1/2 months before Britain ceases to be an EU member. Any deal that is struck needs time to be approved by the British and European Parliaments.

Conservative lawmaker Nick Boles said there was a growing worry among many UK legislators that Britain and the EU are "trying to run out the clock" in order to stymie opposition to their plans.

"They are trying to leave this so late that they can credibly say there is no alternative but a 'no-deal' Brexit, and most people agree that would be chaos," Boles told the BBC.

AP