EU citizens in Britain could be denied access to benefits such as council housing and social security payments after Brexit, a report has warned.
MPs are debating ending EU nationals' right to live and work in the UK.
But Parliament's human rights committee says new laws could leave EU nationals, including those who have paid UK taxes for years, in a "precarious" situation.
The Home Office says the government has already committed to protecting the rights of EU citizens in the UK.
"We want them to stay and, whatever the outcome of the ongoing discussions about our exit from the EU, we will protect their rights and ensure they get the UK immigration status they need," a spokeswoman said.
Theresa May has said EU citizens in the UK will be able to stay even if Britain leaves the European Union without a formal withdrawal deal. They would also keep their social security rights.
EU nationals with a right to permanent residence, which is granted after they have lived in the UK for five years, should not see their rights affected.
However, MPs and peers on the human rights committee have raised concerns that the Immigration and Social Security Co-ordination (EU Withdrawal) Bill could leave people in a "rights limbo".
"Although the government has said that it is not its intention to strip EU citizens resident in the UK of their rights, that is the effect of this bill as it stands," the report says.
'Not good enough'
The committee has urged ministers to build in guarantees to ensure EU citizens will be entitled to the same rights as now.
According to the report, the bill as it stands relies on the home secretary enacting secondary legislation - laws created using powers given to ministers by Parliament - to restore the same rights that people from EU countries have at the moment.
The committee adds that it is proposing amendments to the legislation so that current protections and guarantees can be enshrined in law.
Its chair, Labour MP Harriet Harman, said: "We're talking about the rights of people who have resided in the UK for years, decades even, paying into our social security system or even having been born in the UK and lived here their whole lives.
"Promising that everything will be worked out in the future is not good enough, it must be a guarantee."
The committee also highlighted concerns about the settlement scheme for EU nationals, notably around the time limit and the lack of a physical proof of status.
The Home Office spokeswoman said the settlement scheme was designed to be "as simple and straightforward as possible" and that the government had launched a nationwide marketing campaign to encourage EU citizens to apply.