The number of Romanians and Bulgarians working in Britain has gone down since border controls on them were fully lifted in January, according to the Office for National Statistics (ONS).

The first official numbers of Romanians and Bulgarians in the UK workforce undermine predictions that hundreds of thousands would come to Britain once the doors were fully open to them. One ex-Ukip MEP even claimed 1.5 million would come looking for work.

The Liberal Democrat chief secretary to the Treasury, Danny Alexander, said the figures "give the lie to Ukip's scaremongering on immigration", while the chairman of the Commons home affairs committee, Keith Vaz, claimed that those who "predicted the end of the world on 1 January" – such as Nigel Farage – now owed the public an apology.

Romania's ambassador to the UK, Dr Ion Jinga, wrote on the Huffington Post UK website: "The Romanians' flood to the UK is over even before it started."

The labour force survey figures show that there were 122,000 Romanian and Bulgarian nationals working in Britain in March this year – a fall from 125,000 in December, just before the last of the seven-year transitional controls were lifted on the new EU members on 1 January.

This was 19,000 more than in March 2013, showing that the numbers increased by 20% before the controls were lifted. During that time, self-employed migrants and those working for multinational companies were able to come to work in Britain.

The fall may be accounted for by Romanians and Bulgarians deciding to work in other EU countries such as Germany – which also lifted final border controls on the two countries on 1 January – rather than come to Britain.

The ONS also published figures for the UK workforce broken down by country of birth, which includes long-term settlers and people who are now British citizens. By this measure their numbers also fell and were down by 4,000 in the first three months that controls were lifted but were up by 28,000 on the same period 12 months earlier.

The overall labour force survey figures show British workers have taken more than 75% of new jobs in the UK economy during the past 12 months. UK nationals took 563,000 of the 741,000 extra jobs that had been created, compared with the 178,000 that went to foreign nationals.

The detailed figures show that there are now 2.7 million foreign nationals in the UK workforce of 30 million – just under 10% – which includes a rise of 74,000 in the first three months of 2014. This is almost entirely accounted for by an increase of 75,000 workers from eastern Europe, including Poland.There will also be 60,000 fewer short-term workers from Romania and Bulgaria coming to Britain this year after the closure of a seasonal agricultural workers scheme.

Ukip responded to the figures by ignoring the fall in Romanians and Bulgarians in the UK workforce. Farage tweeted: "Huge increase of 292,000 foreign workers in past year demonstrates that the coalition immigration policy has been an abject failure."

Patrick O'Flynn, Ukip's head of communications and a candidate for the European parliament, said immigration was still "completely out of control" and the party was "not backtracking an inch" on its claims about Romanians and Bulgarians."[This is] based on one quarter's figures on which no statistician would base predictions on," he told the BBC's Daily Politics. "We didn't talk about one quarter's figures, we're talking and still predicting a very big long term rise as part of a much bigger EU rise.

"It's now two-thirds of the increase is from the EU... Every sign is it is going to go up and up and up. That's a betrayal of the British people and we are not backtracking an inch … We are going to have hundreds of thousands of Bulgarians and Romanians".

Sir Andrew Green, of the anti-immigration pressure group Migrationwatch, also dismissed the fall in the Romanian and Bulgarian figures, saying: "These quarterly statistics always fluctuate. The latest annual figures show an increase of 28,000 in a period when restrictions were in place for nine months," and added that he stood by his prediction that 250,000 more would come over the next five years.

But Vaz said: "Those, including Ukip, who promised the end of the world on 1 January, now owe the public and those from Romania and Bulgaria a full apology."

He said the figures were unsurprising to him as he had been at Luton airport on 1 January and seen how the supposed flood was little more than a trickle.

"It would appear now that many may have actually left the UK. There is a clear need for an estimate to be produced on migration whenever EU countries accede.

"By not understanding the likely levels of immigration we risk increasing the poisonous rhetoric and prejudice which leads to the destruction of all rational debate. We must not have an immigration arms race."

Barbara Roche of the Migration Matters Trust and a former Labour immigration minister, said: "Today's figures show it would take 90 years for migration from the EU to get near the 26 million figure Ukip have used in their poster campaign."

In November, Mark Harper, the then immigration minister, said there would be no mass migration of Romanians and Bulgarians to Britain when the labour market curbs were lifted in January. He said there would not be repeat of the mass arrival of Poles in 2004 and suggested people were more likely to go to work in Germany, Italy or Spain than come to Britain.

A BBC Newsnight poll of Romanians intending to work in another EU country last April showed 30% wanted to go to Italy, 24% to Germany and 16% to the UK.

• This article was amended on 14 May 2014. It originally stated that Keith Vaz was at Stansted airport on 1 January.