Adult colouring books could be subject to VAT under new plans being considered by the government.

Currently, both adult and children's colouring books are exempt from the VAT.

However, in future, colouring and dot-to-dot books for adults could be classed as "uncompleted" books, which attract the full 20% rate of VAT.

More than three million adult colouring books were sold in the UK in 2015, worth an estimated £20.3m.

HMRC confirmed to the BBC that it planned to speak to publishers about the rules regarding VAT on adult colouring books, but stressed rules regarding children's colouring books would remain unchanged.

"There's been no change to the rules. Children's colouring books are entirely free of VAT and there are no plans to change that," an HMRC spokesperson said.

"We are meeting with publishing representatives shortly to discuss the VAT treatment of adult colouring books."

While literary and reference books are currently exempt from tax, the government website states that certain other types of books should be standard-rated.

Stamp albums are subject to VAT, as are books which are classed as stationery, such as diaries and address books.

James Daunt, managing director of Waterstones, told The Bookseller: "There needs to be clarity from HMRC around the issue on the definition of books which are VAT-exempt because there is lot of confusion at the moment.

"Retailers will be affected and will owe VAT on their margin of the sale, along with publishers, and if it is confirmed we owe HMRC that it will certainly be a sizeable chunk of money."

However, Daunt added the company had plans in place to pay for it.

Adult colouring book have grown increasingly popular in the last year, with titles such as the Harry Potter Colouring Book and Johanna Basford's Enchanted Forest among the bestsellers.

Basford's publisher Laurence King has decided to VAT-rate her books while it awaits the result of discussions with HMRC, according to The Bookseller.

He told the magazine he "strongly" supports the position of the Publishers Association that all colouring books should be classified as books, not stationery, and so should not be subject to VAT.

"It seems illogical that, for example, crossword puzzle books and Sudoku should be VAT exempt but adult colouring books are not," he said.

"To simplify the situation for retailers… going forward [we] will be publishing under the assumption that colouring books aimed at adults are VAT-liable for the time being, with their liability to VAT subject to the outcome of the discussions with HMRC."