The number of Romanians and Bulgarians to arrive in the UK next year may be as few as 20,000, according to a cross-party lobby group, Migration Matters Trust.

Atul Hatwal, director of Migration Matters, said the estimate – far lower than competing claims by groups such as MigrationWatch – was based on analysis of historical EU migration patterns, macro-economic factors such as the impact of the recession on the UK, and limited job opportunities in areas such as manual labour.

Hatwal said the figures exposed the higher claims as unfounded and inflammatory. "Our belief is that the anti-immigration lobby have cried wolf once too often," he said. "Their claim is that as many as 300,000 new migrants will arrive from Romania and Bulgaria over the course of 2014. In truth we believe that figure will probably peak at around 20,000."

Ministers have so far refused to put a figure on the likely influx, although last week the Office for National Statistics estimated that 37,000 extra Romanian and Bulgarian workers have arrived since June last year, prompting speculation that citizens of the two countries were already preparing for life in Britain even before border controls are lifted on New Year's Day.

A spokesman for Migration Matters said that the figures included Romanians and Bulgarians doing seasonal work in the UK for less than six months, unlike the government's official migration figures which only include people migrating to Britain for at least a year. Hatwal said: "We are fast approaching a defining moment in the debate over immigration in the UK. Organisations such as MigrationWatch and Ukip have consistently argued their anti-migrant campaigns are based on evidence, rather than prejudice

"If 300,000 migrants do arrive from Romania and Bulgaria then the opponents of migration will have shown to be correct. If they don't, then they will have been exposed as doing nothing more than to pursue a partisan and divisive political agenda."

At present Romanians and Bulgarians can work in Britain only if an employer has supported a visa, if they are self-employed or come on a temporary basis for tasks for which there is a labour shortage. Temporary controls imposed in 2007 when the two countries joined the EU will expire at midnight on 31 December, meaning their combined populations, totalling 29 million, will gain full rights to come here to work.