Twice as many Albanians have been caught as stowaways at UK ports than any other nationality, new government figures reveal.

Some 981 Albanian “clandestine migrants” were discovered at UK entry ports from 2008 until spring 2016, according to Home Office statistics released to the Guardian under the Freedom of Information (FOI) Act.

These included a group of more than 50, including a pregnant woman, discovered in a lorry at the Humber sea terminal at Killingholme near Grimsby in June 2015. Albania is not part of the European Union, but borders Greece, an EU country, and its mafia are known to UK police as ruthless people traffickers and drug smugglers.

Afghans formed the second biggest cohort, with 425 port detections during the same period, followed by Algerians (424), Iranians (348), Indians (322), Palestinians (129) and Vietnamese (124). These include clandestine migrants found at UK Eurostar terminals and airports as well as ports and harbours.

One US passport holder was discovered stowed away, as well as one Russian and one, in 2012, from the Czech Republic, which is part of the EU. Despite the free movement of people between EU states, the UK can bar some serious criminals from entering the country.

As the refugee crisis took hold in 2015, there was a big rise in Iranians, Syrians and Iraqis caught entering the UK illegally. In the first quarter of 2016, the last period for which figures are available, there were 122 Iranians detected at UK entry ports, along with 31 Syrians and 21 Iraqis. During that three-month period 318 clandestine migrants were found at UK ports – compared with 556 in the whole of 2015, and 681 in 2014.

Most illegal immigrants are caught before they arrive in the UK, with lorries at Calais and other continental European ports x-rayed for stowaways. Yet the new figures show that hundreds each year still make it onto UK soil before they are detected. In one of the most successful border force operations, 68 migrants were discovered in the back of four lorries at Harwich in Essex, including two pregnant women and 15 children, in June 2015.

The Home Office appeared reluctant to release the information, only partially submitting to the Guardian’s request after they were ordered to comply with the FoI act by the information commissioner.

They refused to comply in full to the Guardian’s initial FoI request, made in May 2016, which also asked at which UK port, train station or airport clandestine migrants had been detected.

Officials from the Home Office considered the information to be exempt from disclosure “on the grounds that release would or would be likely to, prejudice the operation of the immigration controls”.

In their response to the Guardian, an official accepted there was a public interest in revealing the information. “The Home Office recognises that there is a general public interest in openness and transparency in all aspects of government. It is acknowledged that there is public interest in locations where clandestine migrants were discovered. Disclosure of the information requested would result in greater transparency and accountability around operational matters. The release of information would enhance the understanding in Border Force operations at ports of entry, such as Heathrow and Hull,” they wrote.

But they concluded that the information should be kept secret because it would help illegal immigrants and people traffickers to evade border controls.

The Home Office also refused to disclose how many of the clandestine migrants detected at UK ports claimed and received asylum, saying it would cost too much to collate the information.

Information about clandestine migrants can fall into one of three categories: (a) those stopped at the juxtaposed controls (where checks take place on certain cross-channel routes before boarding, rather than disembarkation), (b) those stopped at UK ports and (c) those picked up in-country.

The latest figures released to the Guardian only cover monthly port detections of clandestine migrants in the UK.

Most clandestine migrants are discovered trying to board boats and trains bound for the UK, rather than on UK soil. Information released under FoI laws to the Economist last year showed the number of detected clandestine entry attempts to Britain via European ports and train tunnels increased from about 1,000 a month in 2008-12 to 2,000 in late 2013, before rising to 4,000 in 2014 and almost 13,000 in July 2015.