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Westminster has been agog ever since the revelation that a parliamentary researcher has been arrested under the Official Secrets Act, under suspicion of working as a Chinese agent.

But should parliamentarians have been surprised?

When Ken McCallum, the head of MI5, he warned that the Chinese Communist Party is "playing the long game".

Not only do they want to influence "prominent parliamentarians from across the political landscape," he said, "but people much earlier in their careers in public life, gradually building a debt of obligation."

A strong clue about what Mr McCallum was getting at came last weekend, with the revelation of the arrest.

The Times newspaper suggested another example of China attempting to influence those at the lower rungs of politics, claiming that .

The story has neither been confirmed nor denied by the Conservatives, with a spokesman saying that "when we receive credible information regarding security concerns over potential candidates we act upon them."

The newspaper did not publish any details of the potential candidates in question, or how far through the candidate selection process they progressed.

Two senior sources confirmed to the ´óÏó´«Ã½ that that security services had occasionally warned the Conservative Party to "be careful" about individuals attempting to get on in politics.

But they said these warnings are rare, as well as typically being vague about the reasons for suspicion about the people concerned. The security services do not systematically "vet" Conservative candidates - instead any contact is proactive on their part.

Separately, the ´óÏó´«Ã½ has been told that senior government officials have been warned not to discuss sensitive work in pubs around Parliament for fear that agents of hostile states are eavesdropping.

One individual said that they had been warned that in some packed establishments around Westminster "you just don't know who's there", and that gossip about politicians or officials "could be valuable in a foreign influence operation".

They said that the warnings were not about parliamentary researchers themselves being foreign agents, but that others nearby who appear to either be parliamentary staff or tourists might in fact be spies.