Britain is being “swamped” by immigrant workers, with towns “under siege” from EU migrants, the Defence Secretary has said.

Speaking ahead of the crucial Rochester and Strood by-election, Michael Fallon confirmed that the government plans to introduce new measures to limit the amount of immigration to Britain, possibly even restricting the number of National Insurance numbers given to low-skilled EU migrants.

Speaking to Sky News, Fallon said: “That is still being worked on at the moment to see what we can do to prevent whole towns and communities being swamped by huge numbers of migrant workers.

“In some areas, particularly on the east coast, yes, towns do feel under siege from large numbers of migrant workers and people claiming benefits. It is quite right that we look at that.

“The original treaty when it was drawn up 50 years ago did not envisage these vast movements of people, and we are perfectly entitled to say this needs to be looked at again.

“We will put our proposal forward and we will look for support from other member states as well, including Germany.”

However, the Defence Secretary appeared to climb down later, with a government source saying: “He accepts he should have chosen his words better. He should have said ‘under pressure’.”

The Conservative Party has been attempting to strengthen its rhetoric on immigration after two of its MPs defected to UKIP. Douglas Carswell and Mark Reckless both joined the anti-EU party, forcing by-elections in their constituencies to seek a renewed mandate from voters.

Carswell won his by-election earlier this month, giving UKIP its first elected MP. Reckless will face his by-election next month.

The Conservatives worry that if they do not defeat Reckless in the Rochester and Strood by-election, it could encourage more MPs to defect to UKIP.

The party’s plan to look tough on immigration was dealt a serious blow over the weekend, however, as German Chanellor Angela Merkel said there was no chance of a radical change in EU free movement rules.

“We must not interfere with the fundamental principles of free movement in Europe,” she said.