David Cameron has come under pressure from Brexit-backing senior ministers over the scale of immigration from the European Union.
Culture Secretary John Whittingdale challenged the Prime Minister to release figures which may point to higher levels of migration to the UK than official data as he warned public services were "creaking at the seams" because of the "influx" from the continent.
And employment minister Priti Patel said the UK had been "too tolerant" of immigration from the EU and the Government had "no control" over the number coming to the country.
Mr Whittingdale used a Daily Telegraph interview to release details of the active National Insurance numbers being used by EU citizens in the UK.
Official figures suggest that 257,000 EU migrants came to Britain last year, but over the same period 630,000 NI numbers were issued to EU citizens.
In the Commons on Wednesday, Mr Cameron said the reason why the numbers "don't tally" is because you can get an NI number for a "very short-term visit" and migrants already in the country can apply for one.
Mr Whittingdale said: "There is already enormous concern on the basis of the numbers that are published. The suggestion that they may understate the position is a cause for even greater concern.
"I have heard the reasons why National Insurance numbers don't necessarily reflect actual levels, but at the very least that's a debate which we need to have and I can see no reason why we can't have the figures."
He warned that the number of migrants coming to the UK put pressure on housing, education and health.
"It is creaking at the seams," he said. "There is a very strong feeling that this is a small country and we simply cannot go on having an enormous influx over which we have no control."
His concerns were echoed by Ms Patel, who told the Daily Mail: "At the moment, we have no control. People move here from accession states, putting pressure on all our public services. School places is a classic example.
"We've become too tolerant. We've just sat back and accepted it."
Pulling out of the EU would allow the UK freedom to spend the money it currently sends to Brussels.
"You could build a hospital or a school with that. Think of all the roads we could build. The potholes we could mend. All the local services."
She also raised concerns about the impact of EU regulations on domestic appliances, claiming Brexit could mean "looking at a future with really powerful hairdryers, light-bulbs and vacuum cleaners".
"If you're like me and you need a hairdryer powerful enough to dry your hair in five minutes flat at 20 minutes past six in the morning, well, soon we might be able to get on again."
The interventions by the two ministers, both linked to the Vote Leave campaign, came as the rival Grassroots Out (GO) movement planned an "action day", taking the Brexit message to high streets across the country.
Ukip leader Nigel Farage said: "The grassroots will play a major role in this referendum campaign. Saturday's GO action day will be the first of many.
"The ground campaign will be vital to winning over wavering voters and getting our country back. We are proving that, by bringing people together under the GO umbrella, we have the troops needed to win."
Labour former minister and GO campaigner Kate Hoey said: "The referendum will be won town by town, street by street, house by house."