Banksy's latest creation is one for the records

Security has been increased at a central London branch of HMV after one of Britain's most bankable artists secretly installed two original artworks there.

Graffiti artist Banksy placed the works – a painting on a canvas and another stencilled on a rusty oil can – in the Oxford Street branch of HMV earlier this week.

Staff had been unaware that the artist, whose works sell for tens of thousands of pounds and have been collected by Angelina Jolie and Kate Moss, was leaving his creations. Instead they were expecting a window display of posters plugging a DVD release of his film Exit Through The Gift Shop.

The 8ft by 4ft canvas features a baseball-capped child praying before a paint-daubed stained glass window. It appeared overnight and was found by staff on Monday morning after they let Banksy's team in to prepare the display.

Spray cans and brushes were left nearby, suggesting the image may have been prepared on-site. The works have already started to draw crowds and a security guard has been hired to protect them.