The Office of the Scottish Charity Regulator is "strongly urging" any English or Welsh charities that operate in Scotland but have not yet registered with it to do so immediately.
Under the Charities and Trustee Investment (Scotland) Act 2005, all organisations that want to represent themselves as charities in Scotland and have a "significant presence" there - such as fundraising, occupying premises or carrying out charitable activities - have to register with the OSCR.
An OSCR spokesman said the original deadline for registration was 22 February 2007, but the regulator was aware that a small number of charities had not met it.
"Where such organisations have taken steps to apply within a reasonable timeframe, we will take no further action," he said.
But he warned that the OSCR would soon be taking active steps to ensure compliance.