The leader of Brent's Liberal Democrats had demanded that the borough's refuse collectors are fined, claiming that operatives have broken a contract condition and asked for Christmas tips.

Councillor Paul Lorber said that workers from Onyx Environmental Services had been seen knocking on doors in Sudbury "with the clear intention of collecting tips" as early as 7.15am on December 19, despite it being a condition of their contract that they never ask for gratuities.

He has demanded that a fine of £20,000 should be imposed, and that the money should be donated to the Mayor's charity.

Councillor Lorber said: "It's clear from the contract that residents should not be bothered for tips, at Christmas or any other time, and certainly not at 7 in the morning.

"I also think it's a bit of a cheek, given Onyx's failures throughout the year.

"I've therefore asked the council to fine Onyx, as a company, £20,000 for breach of contract - and for this money to be given to charity.

"I think it would be entirely appropriate given the circumstances."

A spokeswoman for Brent Council said: "If it's proven that they have been asking for tips, the council will then fine them and Onyx will carry out their own investigation into it."

She explained that it was very unlikely that the fine would be for as much as £20,000. "The fine amount will be worked out by calculating the time a council worker takes to look into the problem."

Christophe Bellynck, director for Greater London at the Onyx Environmental Group said: "We have issued a memorandum reminding staff that they should only be approaching houses to knock on doors if a resident has forgotten to place a bin out for collection.

"Onyx does not condone the practice of approaching residents to ask for discretionary Christmas tips and we will be carrying out an internal investigation to determine whether this has been happening in Brent with our own employees."