Conservationists have launched a global campaign asking the public to help tackle the illegal trade in wildlife.

They have developed a smartphone app that allows people to submit images and data of suspicious items on sale, possibly helping enforcement agencies.

The United Nations estimates the illegal trade is worth billions of pounds each year.

Despite efforts to crack down on the slaughter, it continues to grow, say campaigners.

In recent years, there has been a resurgence in the poaching of threatened species, raising concern about the long-term survival of iconic animals such as tigers and rhinos.

Growing demand for protected animal body parts and products on the Chinese black market is widely viewed as one of the main drivers for the growth in the trade.

A recent report by UK think-tank Chatham House said demand was rising at an "alarming rate".

The authors said that activity in the illegal ivory trade had more than doubled since 2007, with ivory reaching a price of US $2,205 (£1,526) per kilogram in Beijing.

Rhino horn was reaching mind-blowing prices of US $66,000 per kilogram - more than the price of gold or platinum.

Crime prevention agencies recognise the threat posed by criminal targeting wildlife - listing the global trade alongside drugs, arms and human trafficking.

The app - Wildlife Witness - was developed by Taronga Conservation Society Australia in partnership with Traffic, the wildlife trade monitoring network.

Its focus was the South East Asia region, which has been identified has a hub in the global illegal wildlife trade.

In this expansion of the scheme, Chester Zoo will look to raise awareness of the project across Europe while San Diego Zoo will do the same in the US.

"The reason why it is important for zoos to get involved is because we have access to really large audiences, and zoos have an important role to try and get these messages out," said Scott Wilson, head of field programmes at Chester Zoo.

"Rather than us just saying 'Look at this, isn't it terrible?' this campaign allows people to download the app and actually be involved and take action."

 Wilson told BBC News how people could submit data: "If people are travelling or are on holiday and if they see something as they are walking through the markets and they see something - say a baby sun bear that should not be there, or ivory on sale that they suspect is illegal, they can record it with this app and the data goes straight to Traffic.

"This will really boost the amount of information that is coming through to them, and this helps them to identify trends in wildlife trade and - more importantly - they can use that data to try and influence the enforcement policies and the really big changes that need to take place."

However, Wilson was quick to point out that people should not take unnecessary risks.

He urged: "We do not want people putting themselves at risk and we certainly do not want them confronting people in a marketplace. If they can take a picture, great, if not then just put the data in the app afterwards."

Chris Shepherd, South-East Asia regional director of Traffic, said there had been an "unparalleled spike in illegal wildlife trade".

He observed: "Sadly, animals are being illegally killed or taken from the wild around the world to be sold for traditional medicines, luxury food, horns or other parts, restaurant dishes, fashion items or pets."

Dr Shepherd explained how people could help using the app: "We want people to be the eyes and ears in the fight against the illegal wildlife trade."