Up to around five staff from the Income Tax Department of India entered AZB and ALMT offices unannounced on Thursday and Friday respectively.

It is understood that the team remained at each firm for several hours, requesting information on clients, particularly in respect of offshore tax structures.

It is understood that the law firms cooperated with the Income Tax Department's requests where possible.

A source said: "They were very cordial and they got it over very professionally."

A direct tax expert with one of the Big Four accountancy firms told Legally India: "I have not really heard of law firms getting raided in the past – I would not say it's a routine event."

He speculated that the tax authorities have become far more aggressive after challenging the Cayman Islands offshore structure in Vodafone's acquisition of Hutchison Essar, which landed Vodafone with a $2bn capital gains tax bill.

"They are now looking at other similar transactions so they could hold the buyer responsible for income tax."

The accountant explained that generally one would have to comply with the tax office's directions: "They have the powers and they can issue summons asking for a particular person to appear before them in relation to a particular case and they have wide powers to search and survey.

"Of course one may be able to litigate and defend but it depends on what authority they carry and what kind of information they have."

ALMT and AZB declined to comment.