EU citizens have until midnight to apply for post-Brexit residency in the UK, with campaigners fearing that thousands will be left behind.
Those who do not apply for so-called settled status will lose legal rights and could leave themselves vulnerable to deportation.
British ministers have praised the scheme as a success after more than five million EU citizens applied for the status, far more than the government expected.
But critics fear a repeat of the Windrush scandal in which Caribbean migrants who moved to Britain after the Second World War were threatened with deportation decades later because of missing paperwork.
At least 300,000 EU citizens are thought to be in limbo because they are still waiting for a decision as the deadline looms.
Home Office figures obtained by the Institute for Public Policy Research, a think tank, show that many applicants were waiting months for a decision.
As of last month, there were 102,000 applications that had been outstanding for at least three months.
About 8,000 people had been waiting a year or more for a decision from the immigration authorities.
The final days before the deadline have included reports of long online queues and people struggling to get help with their application.
Campaigners say that others, including vulnerable children, risk falling through the cracks because they did not realise that they needed to apply. European embassies are urging their citizens to apply in time.
All EU citizens, except those from Ireland, who moved to Britain before the end of last year have been told to apply for the status.
Politicians from the opposition Labour Party, including London Mayor Sadiq Khan, have called for a deadline extension to deal with the backlog.
“There is a real risk of a new Windrush-type tragedy in the future if we don’t get this right now,” said Labour MP Paul Blomfield.
Ian Blackford, the leader of the Scottish National Party in the UK Parliament, said EU citizens were “living in fear for their jobs, their families and their livelihoods”.
But Prime Minister Boris Johnson rejected calls for an extension and said applications were being processed as fast as possible.
He said the government had funded 72 organisations to help vulnerable EU citizens to understand their rights.
“It’s five years now since the Brexit referendum,” he said. “The most important thing is for anybody who still hasn’t applied to get their application in today”.
Anne-Marie Trevelyan, Minister of State for Business, described the scheme as a “very efficient process” and said her own mother had used it successfully.
“Sometimes government processes can be difficult, this one was a really effective and slick one,” she said.
When Britain was a member of the EU, citizens from the bloc could move to the UK without needing to register their status.
It meant that the government did not know how many EU citizens there were in Britain and how many might apply for settled status.
An official estimate of three million EU citizens proved to be significantly short, with 5.6 million applications submitted so far.
The largest numbers of applications have come from Polish and Romanian citizens, followed by people from Italy, Spain and Portugal.
Those who have lived in the UK for at least five years can obtain the full settled status which grants permanent residency.
More recent migrants can get pre-settled status, which lasts five years but can be converted to full residency when they become eligible.