An office worker suspected of stealing donations from the charity where she worked for 28 years was caught when colleagues planted cash in the post, a jury heard.

Susan Castle, 53, is accused of stealing from Sight Savers, an international charity based in Haywards Heath, which works to prevent blindness and rehabilitate blind people in developing countries around the world.

The charity, founded 50 years ago, receives donations of about £20 million a year, including cash sent anonymously to the head office in Bolnore Road, where Castle worked as a legacy officer opening the post.

Bartholomew O'Toole, prosecuting at Lewes Crown Court, told the jury Castle was accused of systematically stealing anonymous donations for a year before she was arrested in November 2003. He said she was allegedly caught stealing a total of £90 after colleagues planted bank notes. But she could have stolen "thousands of pounds" of anonymous donations.

On one occasion she allegedly paid her share of the cost of a staff night out with stolen money.

The court heard Castle's manager, Carol Maughan, became suspicious after noticing Castle leaving the office with envelopes from work in her handbag. On October 14 Miss Maughan placed a £5 note in a pile of post to look like an anonymous cash donation. She later saw the envelope on Castle's desk but no donation was recorded in the charity's accounts.

Two days later two colleagues prepared five dummy envelopes containing cash totalling £55 in piles of post Castle was opening. But the cash was never recorded as donations.

On October 31 a £5 note put in Castle's post was again not logged as being received. Three days later, Castle paid £17 towards the cost of a staff night out. When Miss Maughan later checked through the cash she discovered Castle had paid using the same £5 note.

Mr O'Toole said: "It should never have been in her purse. It should have been in the charity's office. Alarm bells were really ringing by this time."

The following day, three senior members of staff waited until all other staff left the office at the end of the day and planted three notes, totalling £25, in separate envelopes to appear as anonymous donations.

The next day, November 5, they were not recorded as being received after Castle dealt with the post.

When Castle left the office at lunchtime three police officers were waiting for her. They searched her handbag and discovered the three bank notes as well as other cash in envelopes addressed to Sight Savers.

Castle was interviewed at the police station where she denied stealing and said the envelopes were in her bag because she did not want to leave them on her desk during the lunch hour.

Mr O'Toole said after Castle was sacked following her arrest, the number of anonymous donations received at the office increased tenfold.

Castle, of Gatesmead, Haywards Heath, denies five counts of theft.

The trial continues.