The 2011 Census offered an important opportunity to take a snapshot of the UK's total population and help set a benchmark to measure the number of people living in the country. Once again however, questions have been raised over its accuracy.

Westminster Council disputed official Census 2001 figures which claimed that Westminster's population stood at just under 182,000, when other data such as council tax payers, patient registration lists at doctors surgeries and National Insurance numbers all suggested otherwise. After a successful campaign, 17,500 people were eventually added on to the official count. Crucially, if these figures were left undisputed, this could have cost the council an estimated £50m in government funding over the decade ahead.

We are disappointed to be facing a similar situation again. Interim figures published by the ONS in 2010 suggested that Westminster's population stood at 241,100, but the new census figures suggest that  Westminster sustained a dramatic decline in its resident population with numbers falling to 219,400 – effectively the same level that Westminster's population was eight years ago during the 2003-4 estimate.

It would be reasonable to ask where the ONS believes these 21,000 people have disappeared to, and whether the council will have to front yet another campaign to have this portion of our missing population officially included in the final count. We know, for example, there are 4,800 more households paying council tax than outlined in the census figures, and approximately 238,000 patients registered with GPs in the city.

Having an inaccurate population count puts immense pressure on scarce resources. In practice, it means that essential services are underfunded to compensate for extra demand and resources need to be taken from elsewhere. It is imperative that we are able to plan ahead to ensure services can be managed within budgets.

While the Census works well for most parts of the UK, Westminster is one of the most difficult places in the country to carry out a population count. With a third of our population moving in or out of the borough on an annual basis, it is exceptionally difficult for a survey like the census to accurately count the population.