London office development is at a four-year high of 9.7m sq ft, according to the latest Crane Survey by Deloitte Real Estate.

Commercial development is up by 8% over the past six months with construction activity across London having now more than trebled since it reached a low of 2.7m sq ft in mid-2010.

Tenants have also been agreeing more leases on new space - in Q1 2013, 33% of space under construction was prelet - the equivalent to six Shards, a sharp contrast to 2011 when only 1% was prelet.

Cranes continue to dominate the City skyline with nearly 4.5m sq ft under construction, up 10% since the last survey in November 2012.

Leasing activity on the space under construction has almost doubled from six months ago.

Deloitte said 2012 saw the lowest level of office completions in the City for more than 25 years.

This unprecedented low level of new supply has allowed the market to rebalance back in favour of the landlord, as the supply of existing space begins to reduce.

The consequence of the lowest levels of Grade A space for five years is that occupiers are starting to find themselves competing for less space.

Developers in the West End were fast to react to improving conditions following the low level of completions in 2011, and as a result this year will see 1.6m sq ft complete, the highest level for seven years.

Mayfair is set the see the largest influx of space for several years with 400,000 sq ft delivered this year but, according to the survey, the primary focus for new development is the areas of the West End outside Mayfair.

Attracted by Crossrail and infrastructure investment, Midtown now has 722,000 sq ft under construction, the South Bank 982,000 sq ft, and King's Cross 800,000 sq ft.

Deloitte Real Estate partner and head of research Anthony Duggan said: "The increase in construction and leasing transactions recorded in the latest survey reflects the improving sentiment being felt in the London office market.

"Importantly, a number of the 'bellwether' vacant schemes across London are now transacting, reducing the ability of potential occupiers to sit back and wait for conditions to improve.

"This is likely to add a little more urgency into the leasing market over the next few months."