There is nothing to stop the European Commission from "dropping a bombshell" a few days before Britain's in/out EU referendum, MPs have been told.
The EU institution will be banned from donating money to the pro-EU campaign, an Electoral Commission official told a select committee.
It will also be prevented carrying out "promotional activity" during the 28 day purdah period.
But it could publish material, such as a dossier, on its website,
Public Administration Committee chairman Bernard Jenkin said that meant there was nothing to stop the European Commission "dropping a bombshell" days before Britain decides whether to remain in the EU.
Another committee member suggested the European Commission, which has a press operation at Westminster, could even make "vows" to the British people, as Gordon Brown had done ahead of last year's Scottish referendum.
Bob Posner, the Electoral Commission's director of party and election finance, said: "What I couldn't say with clarity is how our enforceability would work outside the UK."
The committee also raised concerns about the government's plan to explain the outcome of David Cameron's EU negotiations ahead of the referendum.
Mr Jenkin said: "I am advised that in no other country could the government use the machinery of government to explain the outcome of the negotiations in that fashion during the purdah period."
He also raised the issue with Lord Bew, chairman of the committee on standards in public life.
The peer said the word "explain" was "open to a number of different meanings" and his committee would investigate whether the government's plans would break impartiality rules.
The committee is holding a snap inquiry into purdah rules on government announcements in the run-up to the EU referendum. It aims to publish an interim next week.
Eurosceptic MPs fear the rules are being amended to allow the government to campaign openly to stay in the EU.
But ministers say it is needed to allow them to continue their work.
The purdah probe will form the first part of a wide-ranging inquiry into the EU Referendum Bill, the proposed law that will authorise a referendum on the UK's membership of the EU by the end of 2017.
Mr Jenkin said he and his colleagues wanted to know why the government was planning for the referendum to partially "disapply" the purdah rules, which normally restrict government announcements in the four weeks leading up to elections.
The inquiry will focus on the existing rules, as set out in the 2000 Political Parties and Referendum Act, the government's case for amending them, how ministers plan to go about it and the impact it will have on the impartiality of the civil service.
In a vote on the issue last month, 27 Conservative MPs rebelled against their party, urging ministers to reinstate the full purdah period although the government won the vote after Labour abstained.
The government has said the existing rules would potentially prevent ministers from attending EU meetings and making decisions with a European dimension. They have insisted they will address MPs' concerns about this and other matters - such as the funding available to different sides and the length of the campaign itself - as the bill makes its way through the Commons.