A Buckingham Palace spokeswoman said: "Princess Margaret suffered a further minor stroke early on Tuesday afternoon.

Princess Margaret recovering at Kensington Palace

"It will be some while before the extent of her recovery can be determined.

"Her Royal Highness is remaining at Kensington Palace where nursing care is being provided.

"The Queen is being kept closely informed and the Princess's family have visited her."

Buckingham Palace cannot say if it is more or less serious than her previous stroke.

Following her release from hospital in January the Princess cancelled all her public engagements but had been seen out on several occasions and it was assumed she was making a gradual recovery.

Three years ago the Princess suffered a mild stroke while holidaying on the Caribbean island of Mustique and was flown back to Britain for treatment at London's King Edward VII hospital.

Details confidential

The Queen and Duke of Edinburgh were visiting Brighton and Hove on Thursday.

They are due to return to London on Thursday evening before heading for Windsor the following day. There are no immediate plans for the Queen to visit her sister.

The Princess is being assessed by doctors at Kensington but details of her condition are being kept confidential.

It is understood her former husband Lord Snowdon is being kept closely informed of the princess's condition.