BRUSSELS (Reuters) - The EU rebuffed a call from pro-Brexit British MPs for a quick deal on mutual residence rights for British and EU expatriates, telling them on Tuesday it was up to their government to launch full-blown divorce talks.

A Union flag flies next to the flag of the European Union in London, Britain, November 4, 2016. REUTERS/Toby Melville

The exchange of letters exposed simmering irritation on both sides of the English Channel before Brexit talks even start.

In a tart reply to a letter from dozens of lawmakers, mostly from Prime Minister Theresa’s May’s Conservatives, who said that ordinary people were being used as “bargaining chips”, European Council President Donald Tusk said he shared their concern over uncertainty but insisted the remedy lay in London, not Brussels.

May herself is pushing for “an early resolution” to anxiety facing over 3 million EU citizens in Britain and over a million British expats. But she has faced a phalanx of EU leaders, including German Chancellor Angela Merkel and EU Brexit negotiator Michel Barnier, who rule out any negotiation until she triggers the exit process under Article 50 of the EU treaty.

She has irked her partners by holding off doing so since the June referendum vote to leave, but says she will do it by March.

Tusk described the MPs’ argument that by refusing to open negotiations Barnier was worrying expatriates as “interesting”.

“The only problem being that it has nothing to do with reality,” he wrote. “Wouldn’t you agree that the only source of anxiety and uncertainty is rather the decision on Brexit?

“And that the only way to dispel the fears and doubts of all the citizens concerned is the quickest possible start of the negotiations based on Art. 50 of the Treaty?”

Triggering talks will set Britain a two-year deadline to cut a deal it hopes can protect its trade with the bloc while ending free immigration from the EU. Without a deal, it faces a sharp and disruptive exit which both sides say they do not want.

“HOSTAGES”

Over 80 British parliamentarians from the pro-Brexit European Reform Group wrote to Tusk on Friday asking that the EU summit he will chair on Dec. 15-16 “guarantee ... reciprocal rights” for people living abroad on either side of what will be a new EU-UK frontier once Britain has left the European Union.

It was a “technical and administrative matter” that could be resolved swiftly, they wrote: “No European expatriate’s livelihood and family should be held hostage in this way.”

Tusk, a former prime minister of Poland whose citizens make up the biggest group of EU expatriates in Britain, said the Council could only address the matter once London opens talks and argued that only a full negotiation could settle the issue.

In a one-page reply, heavy with irony, he said he welcomed the euroskeptics’ concern for expatriates, “especially” since he had assumed their Brexit vote was motivated by “the rejection of the free movement of people and all the rights it entails”.

“Just like you, I would like to avoid a situation where citizens become ‘bargaining chips’,” he added. “In order for this not to happen, we will need precise and comprehensive solutions, which, other than nice-sounding expressions, will provide citizens with genuine guarantees of security.”

EU officials said the British letter was seen in Brussels as an attempt to flush out responses from EU states and to test the unity they have so far been able to present against London -- notably by tempting Poland and other Eastern members with big expatriate populations in Britain to break ranks to seek a deal.

“They are trying to test the solidity of the EU,” one senior official told Reuters. “They are throwing a fish to see how we react. But we will not go an inch further before Article 50.”