Ten years ago the idea of a firm other than SJ Berwin winning a major instruction for property giant British Land would have seemed unthinkable.

After all, British Land was SJ Berwin's first client and some say it is the reason the firm was created back in 1982.

British Land has always used other firms, such as Berwin Leighton Paisner, Herbert Smith and Nabarro, but, with SJ Berwin's boardroom links, including British Land head Cyril Metliss – the father of SJ Berwin veteran Jonathan Metliss – other firms barely stood a chance of landing any of the trophy work.

But skip forward a few years, several big acquisitions and new management at British Land and, while the relationship still stands, the firm's lead position on every deal is no longer as secure.

An SJ Berwin conflict meant British Land used Freshfields Bruckhaus Deringer when it acquired Pillar Property in 2005 and then subsequently turned to Freshfields again for the corporate aspects of its real estate investment trust conversion – using SJ Berwin for real estate and restructuring advice.

Linklaters, with its experience in tall tower planning in the City, has also been advising the company on its new Leadenhall building, dubbed the Cheese-grater.

The threat of Linklaters is significant. While the magic circle law firm has been advising the company on the tower's planning matters for around six years – based on its work on the Heron Tower and the Gherkin – its relationship has since deepened.

With planning secured, Linklaters is now advising on commercial real estate and construction matters associated with the building. These are instructions that, as one partner at a rival firm says: "SJ Berwin must have been extremely upset to have lost."

And it is not just the magic circle SJ Berwin will need to keep track of. Jones Day has maintained its close links to subsidiary Pillar and, more recently, Ashurst has also been making inroads with British Land. The top 10 City firm's work so far has been on the office and retail park side, including on the sale of a Wembley retail park to Quintain earlier this year.

To be fair, British Land cannot be expected to use SJ Berwin for everything. Much of this promiscuity is not due to a breakdown of relationship with the firm, but more a reflection of the need for a greater variety given the growth of the company.

A partner at one advisory firm says: "This is not a reflection on SJ Berwin. It is just a commercial reality that certain firms have more experience in certain areas. These people are sophisticated users of legal services and it is a sensible way to keep people on their toes."

SJ Berwin says it is not concerned by rival firms encroaching on its territory, pointing to the fact that British Land remains the real estate practice's single biggest client and the firm still carries out the overwhelming majority of its work.

And although the firm's Metliss family ties and links with older partners such as Jeff Smith and Stephen Willson may be getting a little long in the tooth, it insists it has a roster of younger partners building the relationship.

Even so, while no-one is suggesting the long-term relationship will suddenly disintegrate, SJ Berwin will know that Freshfields, Linklaters, Ashurst and Jones Day are formidable opponents and should not be disregarded.