The chances of deporting foreign ex-prisoners arrested today may have been damaged by the Home Secretary's decision to view the dawn raids in person, it has been warned.

Dr John Reid donned a stab-proof vest to witness immigration officers forcing entry to four homes in London, beginning at 5am this morning. Legal experts said his attendance on the raids may allow lawyers for the arrested criminals to question the Home Office’s decision to start deportation proceedings.

Keith Best, the chief executive of the Immigration Advisory Service, said: "It could put him in an embarrassing situation if he was present at a particular arrest of someone over whom he then has to make a disinterested decision about whether they should be deported.

"The Home Secretary should stand above operational matters, to be a disinterested party making decisions in a judicial manner."

Mr Best added: "It could put the Home Secretary in the same sort of personal difficulty that Jack Straw found when he said the Afghan hijackers in 2000 would be on the first plane out, until it was pointed out that he was responsible for hearing their asylum claims and he appeared to have pre-judged them.

"It was tasteless to attend today and the Home Secretary needs more than gimmicks to sort out the Immigration and Nationality Directorate."

Pierre Makhlouf, an immigration lawyer, said: "The issue is whether the Home Secretary’s presence did lend itself to some bias in the Home Office. If this were a case of mine, I would look to see if his presence, combined with a weak or rushed decision, had had an impact on the case."

A Home Office spokesman said officers watched by the Home Secretary made two arrests, while another team, earlier given a "rallying call" by Mr Reid, detained a Jamaican national who had been released from jail for manslaughter.

It is highly unusual for a politician to take part in such a raid, but is typical of the new Home Secretary’s determination to appear "hands on" and live up to his reputation as Prime Minister Tony Blair’s "enforcer".

Mr Reid was brought in to run the Home Office after it emerged last month that 1,019 foreign prisoners - including murderers, rapists and paedophiles - had been freed from jail without being considered for deportation.

A spokesman for the Home Office said four arrests were made today, all of whom were taken to immigration detention centres for processing. He declined to answer whether the Home Office had taken legal advice over whether Mr Reid should attend today’s operation.

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