The Foods Standards Agency (FSA) found that 43 out of 145 samples of lamb takeaways were wrongly described.
The FSA said 25 of the samples were found to contain only beef, which is cheaper than lamb.
Food inspectors have been analysing fast food across Britain and found that lamb had routinely been replaced with pork, beef, chicken, turkey.
The studies by FSA and another by consumer group Which? come more than a year after the supermarket horse meat scandal and suggest food fraud is still widespread.
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Takeaway owners are also being warned that they can be fined up to 5,000 pounds for mislabelling food.
"Prosecutions have taken place against business owners for mislabelling lamb dishes, but the recurring nature of the problem shows there needs to be a renewed effort to tackle this problem," chief operating officer at the FSA, Andrew Rhodes said.
"Clearly the message isn't getting through to some businesses," he said.
The consumer organisation Which? found an even higher instance of contamination, after a series of tests in the British capital London and Birmingham, an ethnically and culturally diverse city with a large South Asian population.
In a similar experiment in London, meat in eight of the samples was not pure lamb.
"The government, local authorities and the FSA need to make tackling food fraud a priority and take tougher action to crack down on the offenders," said Richard Lloyd, the executive director of Which?
Which? also wants the government to implement some of the recommendations in the Elliott Review, which followed last year's horse meat scandal.
Among Professor Elliott's 48 interim recommendations, he suggested setting up a food crime unit, to police food standards better.
Earlier this week, the FSA also announced a new round of testing on beef products, to check for horse meat. The tests have been ordered by the European Commission following last year's scandal.