British in Europe, which represent the 1.2 million UK ex-pats living on the continent, told the EU Future Relationship Select Committee on Tuesday that, for many, the loss of rights was "already real".
UK nationals currently have the right to work and study abroad during this so-called grace space while the EU and Britain work out leaving terms.
But many Brits abroad are apparently being rejected for work and job interviews as employers face uncertainty over changes to movement rights.
Kalba Meadows, a steering group member of British in Europe who lives in France, said: “We are seeing people refused interviews for jobs because those jobs require the freedom to travel across the EU.
“We’re not even at the end of transition yet and there are already real-live instances of people’s lives and livelihoods being affected – and that will only increase.
"So many jobs rely on free movement because of the single market.
“We have a very large number of people whose livelihood is based on working in different countries – people with small businesses, people who are employed, and an awful lot of them face losing our livelihoods. It’s not just about losing your rights on paper, it’s something that affects real lives."
The Brexit withdrawal agreement protects some of the rights of British citizens abroad as well as EU citizens in the UK.
However, many holes in the agreement are throwing uncertainty over the future of workers rights, notably in regards to jobs based across more than one member state.
The integrated nature of EU economies means that travelling across borders for work is common.
Jane Golding, the chair of of the organisation, told the same committee on Tuesday that the suggestion that rights for EU nationals after Brexit were the same as before was misleading.
She said the rights were in fact “broadly the same only in the host country where we are living now".
"So that means that we will keep most of our rights that we currently have in the country where we live now.
"But we will not have any EU-wide rights of free movement, for example, or EU-wide recognition of our qualifications. There are no rights in the withdrawal agreement dealing with cross-border working.”
"We haven’t had any indication from either side that this is a topic that’s being discussed in any detail in the future negotiations.”
Michael Harris, another steering group member of British in Europe who lives in Spain, also told MPs: “There’s a stereotype of a Briton who lives in Spain – there are a lot of retired people in Spain who won’t be affected by the need to go and work or provide cross-border services, but around 60 per cent of Britons in Spain are working-age or below.
"Young Britons in the EU are the people who are going to be most affected by this – to go and study and work.”