In a visionary statement last year, the All England Lawn Tennis Club stressed the importance of its ‘tradition and values’.

Voicing its support for the Modern Slavery Act, it said the ‘fundamental principles’ of integrity and respect should be observed by all those who work directly or indirectly for its suppliers.

More than 6,000 miles away at a sweltering factory in the Philippines, these noble words may have been greeted with some bewilderment.

Inside the Philippines Dunlop Slazenger factory where Wimbledon tennis balls are made

Many of the workers in Bataan – where 54,000 Slazenger Wimbledon balls are made annually for the championships – earn as little as 380 pesos a day (equivalent to £5.42 a day or 67 pence an hour).

It is a sharp contrast with Wimbledon ball boys and girls who receive an allowance of around £200 each – and get to keep their smart Ralph Lauren outfits. Meanwhile well-heeled fans at the championships will be sipping Pimm’s (£26.70 a jug this year) watching players such as Maria Sharapova smashing a Slazenger ball across the hallowed turf.

Workers at the Philippines factory – now owned by a Japanese firm – put in eight-hour shifts, six days a week, with scores more on short-term fixed contracts. Despite the sizzling weather outside (often above 100 degrees in the summer), most of the premises are not air conditioned.

Further down the supply chain for the balls, workers earn even less. A plantation and plant which supplies 60 tons of rubber a month to the tennis ball factory is situated on a dangerous island in the south of the Philippines which is terrorised by a group affiliated to Islamic State.

The Mail was warned it is too hazardous to visit Basilan, meaning it is impossible to be completely sure about the working conditions there.

The Barnsley factory (left) shut in 2002. Pictured left is Rafael Nadal of Spain practicing on court during training for Wimbledon

What is beyond doubt is that until 16 years ago the balls were made closer to home in Barnsley. The factory for Slazenger – a quintessential British brand which has been the official Wimbledon ball supplier since 1902 – was the pride of the South Yorkshire town. It shut in 2002, costing more than 100 jobs, and production was moved to South East Asia.

There, controversial tycoon and Newcastle United FC owner Mike Ashley’s Sports Direct International was in charge of the Bataan tennis ball factory from 2004 until 18 months ago. The Ashley regime – which has sparked a huge row for effectively paying UK staff below the minimum wage – got into a dispute with the Filipino unions when it allegedly reneged on an agreement to give workers a sack of good quality rice a month. They got a smaller amount of poor quality rice instead, they said.

Dunlop Slazenger was sold for £112million to Japanese firm Sumitomo Rubber early last year. But, crucially, Sports Direct kept its prestigious Slazenger brand and reached an agreement with the new owners for them to continue making the tennis balls for Wimbledon, at the renamed Dunlop International (Philippines) factory in Bataan.

A machine operator at the factory told the Daily Mail the Wimbledon balls are made months in advance of the championships. On the All England Club online shop, four Slazenger ‘Wimbledon balls’ costs £9. Sources at the factory say the cost of making them is no more than half of this.

‘For employees who have done ten years or more, the salary range is now 500 pesos (£7.14) to 900 pesos (£12.85) a day, depending on your position,’ the machine operator said. ‘Below ten years, you get much less. The last collective bargaining agreement agreed with the British (the Sports Direct-run Dunlop Slazenger) was three years ago.’

The worker, who asked not to be named, said his union had wanted a 40 pesos a day increase (56 pence) in year one of one agreement, but factory bosses – who reported to Sports Direct – agreed only to a 20 pesos a day rise (about 28 pence).

In other words, factory bosses objected to paying an extra three a half pence an hour. Contract workers are not included in the bargaining agreement and get as little as 380 pesos a day, union sources said.

The machine operator said a manpower agency which recruits on behalf of the factory typically gives workers a five-month contract. At the end of this term, they break it for one day and then re-hire the same person. This means contract workers are not paid as much, cannot join the union and do not get the benefits enjoyed by other staff.

The worker said: ‘They (the British) were exploiting us and getting labour on the cheap. This was happening for years.

‘I like the Japanese (new owners). They give you everything you need to keep you safe. If your job requires you to handle materials, they will make sure that you have got proper gloves, protective equipment.’

Another worker, who has been at the factory for more than 15 years, said: ‘In 2004, when SDI (Sports Direct International) took over, our situation became much worse. It was difficult to feed our families.’

In 2012, things came to a head when factory managers allegedly reneged on the rice deal. Union leader Ismael D. Guinto Jnr said: ‘We were all supporting families – we needed those benefits. We felt angry that we didn’t get them.’

On the island of Basilan, where the EJN Rubber company produces rubber to sell to the tennis ball factory, locals are at risk from attacks by Abu Sayyaf, a bloodthirsty jihadi group linked to IS.

The company’s registered owner, Richard New, admitted that nobody from the factory in Bataan – whether under past or current ownership – had visited his plantation or processing factory to check on production methods because of security concerns.

‘I think they are just scared,’ he added. ‘We always get (extortion) letters, and actually our house has been bombed four times.’

His workers are paid a set amount depending on how much rubber they tap from the trees. On average, Mr New said, a farmer gets about 4,000 pesos (£57.14) a month for harvesting rubber for his company. It is not clear how many days of work they have to do to earn this. The minimum wage in the area is about 280 pesos a day (£4) – significantly less than the basic minimum wage in Bataan.

Christine Sanger, the former senior shop steward at Slazenger’s old tennis ball factory in Barnsley, branded the 67p an hour wages ‘disgusting’. She said: ‘When some firms can find cheaper labour somewhere else there is no loyalty. You cannot blame the workers for accepting such low wages. In the Philippines a lot of the people have got nothing. When you have got nowt a little something – even 67p an hour – is better than nowt.’

Last year’s bumper profits at Wimbledon have allowed the organisers to boost the 2018 prize fund by 7.6 per cent – the men’s and women’s champions will each win £2.25million.

Asad Rehman, of charity War on Want, said: ‘Companies are refusing to take full responsibility for supply chains … while exploiting some of the poorest people in the world. Wimbledon is a global event showcased all over the world and one which attracts huge sponsorship. It must ensure that companies take responsibility for all parts of the supply chain.’

A spokesman for the factory in Bataan said: ‘Please be informed that Dunlop International (Philippines), Inc. has no right over Wimbledon’s Slazenger tennis balls as it is under SDI (Sports Direct International). We will respond only upon instruction by SDI.’

The All England Club declined to respond to a series of questions from the Mail. It would only say: ‘We take any concerns about our supply chain very seriously. Since taking retail, merchandise and licensing in house, we have established close working relationships with all of our partners and suppliers on compliance with the Modern Slavery Act.’

A Sports Direct spokesman said: ‘We sold the Dunlop business, including the factory in Bataan, in early 2017 and therefore we have been unable to corroborate many of the points raised by your investigation. However, we take this subject extremely seriously. Pay and working conditions improved substantially during our period of ownership.’