The home secretary, Theresa May, has been warned that she needs to consider bringing in more eastern European workers from countries such as Ukraine and Russia to the UK or face a sharp rise in fruit and veg prices and job losses in Conservative heartland seats.

The official migration advisory committee (Mac) says British farmers should be able to recruit a sufficient number of seasonal fruit and vegetable pickers in the first one or two years after labour market restrictions on Bulgarian and Romanian workers are lifted in December.

But it says there will be a lack of available seasonal migrant labour in the medium term, which will lead to a rise in labour costs and a 10-15% increase in supermarket prices.

Its report, published on Tuesday, warns that shops and supermarkets are likely to turn to cheaper foreign fruit and vegetables, leading to job losses in the British horticultural industry.

The industry is concentrated mostly in Kent, East Anglia and Herefordshire, and local Tory MPs have already issued warnings to the home secretary.

The seasonal agricultural workers scheme (SAWS), which allows an annual quota of 21,250 Bulgarian and Romanian workers to come to Britain for a maximum of six months, accounts for a third of Britain's seasonal agricultural workforce. The scheme is due to close at the end of this year, and the migration experts expect sufficient numbers of Romanians and Bulgarians will come in the short term. But in the medium term of one to two years they would be increasingly likely to move to other permanent, better-paid jobs in the hospitality, care and construction sectors.

The scheme is crucial to the supply of strawberries, salad, apples and other soft fruit and vegetables to British shops and supermarkets.

David Metcalf, the Mac chairman, said migrant workers were not displacing British labour. He said British workers did work in fruit and vegetable picking but wanted permanent jobs and did not want to be tied to a farm living in a caravan or a pod, as required by many farmers.

"We are not saying British workers are lazy," said Metcalf. He said British employers had made great efforts to recruit British workers.

The migration experts say the home secretary should consider replacing the seasonal workers in the scheme with labour from outside Europe – in particular countries such as Ukraine which has a high number of agricultural students.

Metcalf said a decision to replace the scheme in this way would mean the government taking a decision to protect the British horticulture industry as a favoured sector.

The seasonal agricultural workers scheme dates back more than 60 years.

The Mac report says any decline in the supply of seasonal labour could lead to increased pay, increased recruitment costs and a fall in efficiency in production. Supermarkets told the migration experts there was very little flexibility on price for British fruit and vegetables and they were highly likely to switch to imported produce.

Metcalf said: "Growers, operators and workers told us that the labour supply from Bulgaria and Romania will not immediately dry up following the closure of SAWS – but there could be long-term implications that need to be addressed.

"If growers cannot get the required labour, evidence suggests that a replacement SAWS would help horticulture thrive in the long run, but it is ultimately for the government to decide if this sector is a priority."

A government spokesman said: "We are grateful for the Mac's report and note its conclusion that there is little evidence the supply of seasonal labour will decline in the next one or two years.

"We shall consider the Mac's advice very carefully. However, in general we want to encourage employers to recruit from the resident labour market where possible," he said.