The Prime Minister knows his measures to curb immigration will be ineffective, but cannot say so

It was the creators of the 1980s political comedy Yes, Minister who invented the “politician’s syllogism”. It is a way of thinking which runs: “We must do something. This is something. Therefore we must do this.”

Today, David Cameron and Theresa May felt a particular need to do something. New figures from the Office for National Statistics showed that net migration in 2014 was at its highest level for a decade. With 641,000 people coming into the UK, and just over half that number leaving, net migration was 318,000, a 50 per cent increase on 2013.

That does not fit well with the confident speech that Mr Cameron delivered in April 2011, when he said: “If we take the steps set out today… I believe that will mean net migration to this country will be in the order of tens of thousands each year, not the hundreds of thousands every year that we have seen over the last decades … our borders will be under control and immigration will be at levels our country can manage. No ifs. No buts.”

It will have been a bad moment for the Prime Minister and Home Secretary when they first saw these figures. Just when it seemed that Ukip was floundering after a disappointing election result, these numbers came along like a gift to Nigel Farage. It is only to be expected that the pair resolved at once that they must do something – and gratefully seized on the first daft something that was put in front of them.

It is not that the measures they set out were intrinsically bad, but it is absurd to pretend they will have any significant impact on the numbers coming in. The most highly publicised idea is that the wages of people caught working illegally will be confiscated. Banks will also have an obligation to check that account-holders are in the country legally. The obvious problem here is that people living and working illegally in the UK do not, as a rule, have bank accounts and wage slips. They work cash in hand. It will be interesting to see whether the Government publishes figures in the coming years into how much money has been taken from illegal immigrants, and whether it can show any evidence that anyone has been deterred from coming to the UK as a result.

Last year, the Government embarked on a similar campaign to make illegal immigrants feel unwelcome. Landlords were to be required to check that their tenants had a right to be in the country, and A&E departments were to make patients fill in forms about their immigration status. There were also the notorious “go home” vans. In fact, the numbers who left, either voluntarily or through deportation, did not rise: they fell.

What makes this performance so absurd is that immigration is largely out of the UK Government’s control, under EU rules about the free movement of labour, and that the high immigration figures are a mark of the Government’s success. It was the expanding economy, with its wealth of job opportunities, which drew people here. And the result was not that British workers were put out of work: the number of Britons in work went up.

But crude politics does not allow Mr Cameron to say that high immigration is a consequence of success and the price of EU membership. He must keep up the pretence that it is a fault which can be put right. He will not “cave in”, he promised. With those words, he has put his own credibility at risk.