By Hugh Pym BBC News Business Correspondent

The cockling tragedy has raised the issue of the hidden economy

The Morecambe Bay tragedy has raised questions about the extent of the illegal economy - work and paypackets which are not reported to the tax authorities.

Allegations by Greeks that they were treated as slaves on a flower farm in Cornwall has also highlighted the growing demand for immigrant labour at a time of near full employment, and the potential for exploitation.

AV Produce, based at Dartford in Kent, is particularly busy at this time of year packing vegetables such as spring greens and cauliflower.

Many of the workers are from outside the European Union including some from Latvia.

They are working legitimately under a recently expanded Government scheme which allows 25,000 immigrants each year to come to the UK for seasonal agricultural work.

"It is good - we get much more than we can in Latvia - we are allowed to stay up to six months - I like England," said one worker.

Seasonal harvesting

Jim Hosking, chairman of the Bulbgrowers of Cornwall, is a daffodil grower.

He is pleased that the government has increased the numbers of work permits available and that he can now recruit pickers when he needs them.

Mr Hosking highlights the pressures resulting from highly seasonal harvesting work: "The good news is that there is much less unemployment in Cornwall so there aren't the number of people we require standing about waiting for a job.

Workers are often needed at short notice to pick cauliflowers

"We need about 5,000 people at this time of year quite suddenly and we haven't got large conurbations where we can get busloads of people who can come out and do the work.

"So the gangmasters who bring teams down are essential and by and large do an excellent job of work".

But he was shocked by allegations of "slave" labour at a flower farm near Hayle in Cornwall. Fifty Greek nationals returned home after their wages failed to materialise.

The boss of the firm which brought the flower pickers over said his business was totally legal, and has claimed the workers simply did not want to work

Paperwork requests

The gangmasters, who provide labour in agriculture and horticulture, are obliged to check the work permits of their immigrant workers.

So are the farmers and other employers who recruit from the gangmasters.

But as Sean Levy of the National Farmers Union says this is not always so straightforward.

"If you've suddenly got a crop of strawberries or cauliflowers which you need picking and you need a hundred people at half past six on a cold morning, then checking the sort of paperwork the Home Office envisages makes you think about an easier way of making a living".

If you are talking about the gross value of the (informal) business being conducted then it might well be at the top end of the scale Lord Grabiner

At the other end of the chain supermarkets demand a ready supply of fruit and vegetables for the shelves.

This in turn puts more pressure on suppliers to deliver at short notice and with the lid kept on prices.

Michael Jack MP, chairs the Commons Agriculture Select Committee, which has carried out an investigation of the role of gangmasters.

He highlights a factor which might lead food producers to cut corners.

"The growing pressure of working seven days a week for the supermarkets who at the last minute can change substantial orders - it requires farmers and growers to have a flexible approach as far as labour is concerned to meet those demands on their business.

"Some have strayed into illegal solutions to cut their costs and stay in business".

Big business

No-one knows how widespread the murky world of illegal employment is.

Four years ago the lawyer Lord Grabiner was asked by the Chancellor to investigate what was described as the informal economy. He said it could be worth anything from £2bn to £10bn per year.

"My guess would be at the lower end of that scale - it depends on how you define it," he said.

"But if you are talking about the gross value of the business being conducted then it might well be at the top end of the scale."

Agriculture and horticulture are part of that. As long as UK unemployment remains low, there will be a shortage of workers available to do arduous picking and packing and scope for abuse both of workers' rights and the tax laws.