An amnesty for illegal immigrants would only act to attract more to the UK, according to shadow immigration minister and Kent MP Damian Green.

The MP was responding to a suggestion by London Mayor Boris Johnson that there should be an amnesty.

He spoke after a study indicated the number of illegal immigrants in the country could be as high as 725,000, considerably more than previous estimates.

Mayor Johnson said: "If it does look as though they [illegal immigrants] could make a contribution to society, we should regularise their status or offer them the chance of regularising their status."

While acknowledging there would be political resistance, he argued that if people were already living here it was "morally right that they should contribute in their taxes to the rest of society."

But the idea was swiftly dismissed by the Conservative shadow immigration minister Damian Green.

He said: "I do not support an amnesty. The problem is that they store up trouble for the future because once you have an amnesty, people come to expect there will be more to come.

"The experience of other countries like Spain and Italy is that the long term impact is for it to encourage even more illegal immigrants."

The focus instead should be on establishing a border police force to secure the UK’s entry points and for greater emphasis on cracking down on employers who took on illegal immigrants for work, he added.

"If you can find illegal immigrants through employers, it is much easier and more efficient to remove them. The Government is belatedly trying that but is not being that effective."

The Government also condemned the plan, with minister Phil Woolas saying: "What unfortunately would happen is that people traffickers and others would see that as a pull factor to get people to the United Kingdom illegally.

"We would end up with a bigger problem not just for our society, but for the people themselves involved."