The high number of returnees is five times the number that was previously reported, the Sunday Times reported, and highlights the growing danger of tourists traveling to Syria.
Security officials said they are closely monitoring the 250 returnees, including veterans who have fought in Afghanistan or Pakistan.
Immigration Minister James Brokenshire said the terror threat posed by individuals who have trained in Syria is a big problem. He urged officials to be vigilant about travel between England and Syria, the BBC reported.
His comments come after a video emerged of a British man carrying out a suicide bombing in Syria.
Last fall, police thwarted one serious plot staged by a group of returned militants who were allegedly planning a gun attack on civilians. Militants with suspected links to al-Qaida have been traveling to Syria from many countries since fighting broke out there in 2011.