Timing of DNA replication initiation is dependent on S-phase-promoting kinase (SPK) activity at discrete origins and the simultaneous function of many replicons. DNA damage prevents origin firing through the ATM- and ATR-dependent inhibition of Cdk2 and Cdc7 SPKs. Here, we establish that modulation of ATM- and ATR-signalling pathways controls origin firing in the absence of DNA damage. Inhibition of ATM and ATR with caffeine or specific neutralizing antibodies, or upregulation of Cdk2 or Cdc7, promoted rapid and synchronous origin firing; conversely, inhibition of Cdc25A slowed DNA replication. Cdk2 was in equilibrium between active and inactive states, and the concentration of replication protein A (RPA)-bound single-stranded DNA (ssDNA) correlated with Chk1 activation and inhibition of origin firing. Furthermore, ATM was transiently activated during ongoing replication. We propose that ATR and ATM regulate SPK activity through a feedback mechanism originating at active replicons. Our observations establish that ATM- and ATR-signalling pathways operate during an unperturbed cell cycle to regulate initiation and progression of DNA synthesis, and are therefore poised to halt replication in the presence of DNA damage.