There is mounting concern from business at a move by the Office of Fair Trading to stop giving confidential advice and guidance on mergers.
It recently withdrew - with limited exceptions - both procedures, regularly used by companies to gauge the extent to which a proposed merger that was still confidential might raise competition concerns.
Under the Confidential Guidance procedure the OFT would respond with a written indication of whether a transaction was likely to be referred to the Competition Commission. This has been suspended totally.
The Informal Advice procedure, dubbed the "fireside chat", led to an oral indication from the OFT of whether a transaction was likely to be referred.
This will now be available only in exceptional circumstances.
Birmingham law firm Pinsent Masons said that, theoretically, it would remain possible for advisers to try and persuade the OFT to offer informal advice but the likelihood of that happening was very small.
"The OFT itself states that advisers should work on the basis that informal advice will no longer be possible," it said.
And it went on: "The two procedures were only available when the proposed merger was still confidential. The practical impact is that a merger notification to the OFT will now only be possible when there has been a public announcement."
Guy Lougher, national head of the EU and competition group at Pinsent Masons, said: " Companies will now be obliged either to complete a transaction just relying upon their own advisers' assessment of whether it may be referred to the Commission, or to publicly announce a proposed merger and then pre-notify it to the OFT for prior merger clearance.
"Unfortunately, the latter option is unlikely to be acceptable to many vendors because of risk of their business being adversely affected by the commercial uncertainty created in the period between announcement and the OFT granting merger clearance once it has been announced that the business is up for sale.
"The OFT's announcement will therefore have the practical effect of discouraging transactions that would otherwise have happened or of placing additional commercial risks on purchasers - either outcome is unsatisfactory.
"This change in policy also means that clients will become much more dependent on expert competition law advice, which will increasingly need to be sought as early as possible." n Pinsent Masons is strengthening its international operations with the further expansion of its Far East practice.
It has taken on two corporate partners and a senior China-qualified lawyer from other firms in Hong Kong and China.
The appointment of the two partners, Dale Fischer from Clyde & Co and Andrew Lui from DLA, and consultant Amanda Yao from Boss & Young, complemented by a number of other hires, will increase the size of Pinsent Masons' corporate team in Asia to a total of 14 lawyers.