The investigation centres on suspicions of corruption within horse racing and fixing the outcome of horse races.

It is understood the allegations involve more than 80 races over the last two years.

Of the 16 arrests, eight were made in South Yorkshire, three in North Yorkshire, three in Suffolk and two in Hertfordshire, a police spokesman confirmed.

Detective Chief Superintendent Steve Wilmott, of City of London Police, who are heading the probe, said: “Of the people we have arrested three are jockeys and one is a horse trainer.

“We have amassed a large amount of information, including computer records and documentation seized today, and will now commence the detailed task of examining it all.”

City of London Police Assistant Commissioner Mike Bowron said: “The City of London Police are leading this complex investigation with assistance from police in Suffolk, North Yorkshire and South Yorkshire.

“We are leaders in the investigation of economic crime and are very experienced in dealing with large-scale fraud cases.

“We are very pleased to have taken the lead role in this operation, which spans the whole of the country. This case is of great national significance, not just to the racing community, but to the wider public throughout the UK.”

Those arrested were still in custody and the investigation was continuing, police added.

A South Yorkshire Police spokeswoman said: “South Yorkshire Police are assisting the City of London Police in a joint operation. A number of arrests have been made across South Yorkshire this morning. These persons are being detained for questioning at police stations across the county.”

John Maxse, director of public relations at the Jockey Club, said: “This is an investigation which was initially handled by the Jockey Club, then it was passed on to the City of London police earlier this year.”

Jockeys’ Association chief executive John Blake told BBC News: “It appears very serious indeed.

“The police have amassed a huge amount of evidence which they will now go through, it does seem strange to me on first sight of that, that they have not done that prior to these dawn arrests and we will wait to see the outcome of that.

“Of course it is very damaging to the reputation of racing, very damaging to the reputation of trainers and jockeys potentially and it’s a great concern to us.

“The Jockey Club itself licenses the jockeys and trainers and as a consequence of that they have their own proper disciplinary process to call upon and it seems at this point that this process has been leapfrogged and become a police matter, because to my knowledge no jockeys have been involved in an investigation into this.”

Betting guru John McCririck told ITV News: “This so undermines the integrity of racing and could not have come at a worse time.

“What has brought this all to the surface is the emergence of the exchanges, where you can not only back horses but also lay them to lose. Previously, only bookmakers could lay a horse to lose, you had to have a licence and there weren’t too many of them.

“Now anyone can go on the exchanges and lay a horse to lose for whatever reason. But there is now a paper trail and all the business of the exchanges is opened up to the authorities. In the old days the bookmakers kept very quiet and protected the confidentiality of their clients.

“But it is the image of racing that suffers. We have the position with athletics and with swimming that whenever a record is broken, people think what drugs are they on? That’s what you think of straight away rather than what a fantastic achievement.

“People will be asking was that race crooked and was that horse trying? That has been the cry down the centuries about horse racing but now it has been brought into these lurid headlines once again.”