British citizens in the EU have implored Brussels and London not to make them “a bargaining chip for the second time” in Brexit trade talks.

They are deeply concerned that they are being “massively downgraded” and that they will be used by Brussels as leverage to exact concessions from the UK in the second phase of the negotiations.

Campaigners have written to the EU’s chief Brexit negotiator, Michel Barnier, arguing that their rights should be separated from trade talks and secured urgently.

“Our remaining rights should not be deferred once again,” wrote Jane Golding, the co-chair of British in Europe, in a joint letter with Nicolas Hatton, the co-founder of the3million. “They should not be bargained against, made dependent on or packaged within other negotiation issues. We must not be made bargaining chips a second time.”

They have launched an information campaign for worried Britons because of what they say is a lack of consistency among member states on their rights.

Key among their concerns is the continued right to move and work across Europe. This was not resolved in the withdrawal agreement, which will be formally ratified this week, and was bumped into the second phase of negotiations because the UK wanted to end uncontrolled migration to Britain.

British citizens in the EU feel they are “collateral damage” because the free movement talks have no consequence for EU citizens already in the UK as they have the birthright, as citizens of a member state, to continue to live or work wherever they like in the EU. British nationals in the EU, on the other hand, as citizens of a third country post-Brexit, will not have that right.

It will limit their right to work across borders and remove their automatic right to return to the UK with an EU family member or to return to the UK for a period of time to look after an elderly or sick relative without compromising their residency in the EU.

Golding said: “Free movement is indispensable, not a nice to have, for hundreds of thousands of working people who have rents or mortgages to pay and families to provide for. Without it, our lives are set for a massive downgrade.”

British in Europe said in a separate statement on Monday: “After the transition period they [Britons in the EU] will all be restricted to working in the country in which they are currently resident. For example, a caterer based in the French Alps who currently makes half her income from short-notice jobs in Greece and Spain will not be able to do so in the same way.”

Golding and other campaigners will travel to Brussels on Wednesday to meet Barnier’s taskforce to press their case.

While the rights of British pensioners retired abroad in the EU have been secured in the withdrawal agreement, the campaign group points out that 80% of British nationals in Europe are of working age or younger and do not know what the future holds.

It is concerned that EU member states have not said what system they will use to record the right of British citizens to stay after the end of the transition period.

No other state has launched a special system for them, equivalent to the UK’s settled status scheme. Nor has any member state bar Ireland outlined what Britons will have to do.

If member states go down a similar road to the UK with a constitutive system, there are only 12 months to get it designed, launched, advertised and operational.

“This leaves an extremely short timescale and much uncertainty about the terms of such a system. Many citizens may yet be rejected for the lack of sufficient income or failure to clear other eligibility hurdles,” the group said in the letter to Barnier.

Britons in the EU argue that they moved to another member state in good faith under the rules of the EU and those who moved to the continent before the end of the transition period should retain those rights.